RV Basics: Water Tanks

RV Owners

RV travel is an incredible adventure, and for the most part, it’s all fun and games—that is, until the wastewater tanks need to be dumped. At that point, there are some less fun things that need to be done. 

Unfortunately, there are a lot of people out there who know nothing about RV water tanks, meaning those less-than-fun parts of RVing are a complete mystery to them. That’s where this article comes into play. 

Today, we are going to discuss RV water tanks in depth so you can jump into the world of RVing with all of the knowledge you need to properly use and care for your RV holding tanks. 

water in a tank

How Do RV Water Tanks Work?

First, let’s talk a bit about what RV water tanks are and how they work. Most RVs have a total of three water tanks. One of these is used to hold clean water for showering and washing dishes. The other two? Well, those are made to hold the drained water that comes from showering, washing dishes, and flushing the toilet. 

The waste tanks are mounted to the underside of the RV, and simple piping connects the various drains to the holding tanks. When water is used, it flows down the drain and into the appropriate wastewater tank until it is dumped. 

The freshwater tank is also mounted to the bottom of the RV. It can be filled with clean water, and a pump is used to move the water out of the tank and through the faucets in the RV. Most people take advantage of this tank only when dry camping. 

All of these tanks must be maintained in order to keep working properly. That said, if they are well taken care of, you can expect your tanks to last for many years, making your efforts well worthwhile. 

Freshwater Tank

As mentioned above, the freshwater tank is used to hold clean water, which can be used by those who choose to dry camp (aka boondock) in places where no water hookup is available. This allows those campers to live life normally, washing hands and dishes whenever needed as long as water remains in the tank. 

How Large is a Freshwater Tank?

Freshwater tanks can vary in size. Some are as small as 15 or 20 gallons. Others can be 100 gallons or more.  

If you have a large RV, your tanks are probably bigger than those on a campervan. That said, rig size is not always a reliable indicator of tank size, so you will want to check the specs on any given RV to know the freshwater tank size. 

Obviously, the bigger your tank, the longer you will be able to boondock without going out for a refill.

Using a Freshwater Tank

Obviously, if you’re going to use your freshwater tank, you need to know how to fill it. Fortunately, it’s a relatively easy process, and once you know how it’s done on your RV, you’ll be able to get it done quickly and easily every time:

Close the drain — To start, make sure the drain valve is closed so whatever water you put in the tank doesn’t drain out. 

Fill the tank — Once that’s done, you will need to find the freshwater fill-up on the side of the RV. Generally, this is labeled as the freshwater fill-up, so it should be easy to locate. Connect a freshwater hose (look for a white water hose) to a potable water spigot, remove the cap from the fill port, place the other end of the hose in the fill port, and turn the water on. 

Watch the indicators — With the water running, use your RV’s tank level indicators (usually on a control panel somewhere inside the rig) to see how full the fresh tank is. Once the indicators say the tank is full, you’ll want to turn off the water, remove the hose, and put the cap back on the fill port. 

If the water starts to back up before the tank is actually full, simply turn the water off and let it drain into the tank before starting it up again. You might also try turning the pressure down a bit when you do turn the water back on. 

Only potable water should be put into the freshwater tank. If water that is not potable is accidentally put into the fresh tank, the tank should be cleaned before the next use. 

Cleaning a Freshwater Tank

We recommend cleaning your freshwater tank at least once every six months. That said, many people choose to clean their tanks more often. It all depends on how often you use the tank and whether you accidentally add non-potable water to the tank along the way.

Open all drains — To clean your tank, you’ll first need to turn off the water heater and drain the system by opening all low-point drains as well as the drain on the freshwater tank, and then—after it’s been allowed to cool completely—the water heater tank. When all the water has drained, close these drains and valves.

Fill the tank with bleach water — The next step is to get bleach water into the freshwater tank. If you have an older RV you can do this by pouring bleach water directly into the freshwater fill port. If your RV is newer and won’t allow you to pour bleach water in directly, you can put the bleach into a freshwater hose, connect the hose to a potable water spigot, put the hose into the fill port, and turn the water on to push the bleach into the system. 

Either way, you want to end up with ¼ cup of bleach for every 16 gallons of water your tank will hold. If you will be pouring the bleach in directly, be sure to dilute it a little with a couple of gallons of water before doing so. Once the bleach is in the tank, fill the tank the rest of the way with potable water. 

Pump it through the system — With your tank full of bleach water, the next step is to pump the solution through your lines. To do this, you will simply turn on the water pump and run every faucet and the toilet until the bleach water comes through. At this point, you can turn off all faucets and let the solution sit in the lines and tank overnight, giving it a chance to kill off bacteria. 

Rinse with clean water — The next day, you will drain the tank and lines by opening all drains like you did before. Fill the tank with clean water and run all faucets until you no longer smell bleach coming through. This will leave you with a nice clean tank and system that will give you water that is safe to drink. 

Toilet paper

Black Water Tank

Next in the tank lineup? The black tank. This is the grossest of the three holding tanks, as it is the one responsible for holding the wastewater from your toilet. That said, it is necessary, so let’s make sure you have a good understanding of how to use it. 

How Large is a Black Water Tank?

Black water tanks come in a huge range of sizes. Some pop-up trailers and class B RVs have tiny 18-gallon tanks, while some larger fifth wheels boast 64-gallon black water tanks. 

As is the case with freshwater tanks, smaller black tanks tend to go in smaller rigs, while the bigger ones are often found in larger RVs. Still, this isn’t a foolproof way to know how big the tanks on a particular RV happen to be, so we recommend finding the specs on your specific RV to know your black tank size. 

Since you pretty much have to leave your campsite/spot to dump your tanks once your black tank is full, the size of your black water tank definitely helps determine how long you are able to boondock. To avoid the limitations a black tank can impose, many avid boondockers will install composting toilets, allowing them to avoid having a black tank altogether. 

Using Your Black Water Tank

For the most part, using your black water tank doesn’t take much thought. It gets used automatically when you flush your RV toilet. That said, there are a few things to keep in mind when using this tank:

Use plenty of water — When you flush your RV toilet, be sure to use plenty of water. This helps break down solids in the tank so no clogs develop. It also helps reduce smells. 

Stick to RV toilet paper — Speaking of breaking down solids, you will want to stick to RV toilet paper, or at least very thin 1-ply paper. Anything thicker won’t break down properly, leaving you with a clogged tank. 

Don’t leave the black tank open — If you’re in a place with full hookups, it can be tempting to leave the black tank open so waste can flow directly into the sewer. Don’t do it. This allows all fluids to drain away, leaving solids to sit, dry out, and create a rock-hard pyramid in your tank. Gross, problematic, and frustrating to deal with, all rolled into one.

Dumping a Black Water Tank

Okay, so let’s say you’ve been using your RV for a few days and the black tank is full. What do you do now? While dumping the black tank isn’t a glamorous job, it may not be as disgusting as you’re thinking. 

Connect the hose — First, don some rubber gloves to protect your hands. Locate your dump valves, remove the cap from the end of the dump pipe, and attach one end of a sewer hose to the dump pipe. The other end of the hose should be securely connected to the sewer drain in the ground. If you can’t twist the hose onto the drain, use a large rock (or other suitably heavy object) to hold it in place.

Open the valve — Next, open the black tank valve. This lets the contents of your tank flow through the sewer hose and into the drain. When you no longer hear water running through the hose, close the valve. 

Clean the hose — Make sure the valve is closed all the way, detach the hose from the pipe, and using a water hose specifically for wastewater purposes, spray the inside of the sewer hose clean. Replace the cap at the end of the drain pipe.

To get your tank ready for the next round, put a few gallons of water down the toilet and add a cleaning chemical such as this one

Cleaning a Black Water Tank

Sometimes you will find that your black tank needs a good cleaning. We recommend doing this at least once every six months and before putting the rig into storage. That said, you might find you need to do it much more often. 

Dump the tank — To clean your black water tank, start by dumping the tank as outlined above, but keep the sewer hose connected afterward.

Spray it out — Once the tank is empty, attach a water hose (not your freshwater hose) to the black tank backflush port. If your RV doesn’t have one of these, consider purchasing something like this. Run water into the backflush to remove debris from the tank. It helps to use a clear piece at the beginning or end of the sewer hose so you can see when the water runs clear. 

Put it all away — When the water does run clear, turn off the faucet, remove the water hose, close the tank valve, and remove the sewer hose, making sure to clean it as outlined above. You’ll also want to make sure you replace the cap on the drain pipe after. 

When this is all done, add water and cleaning chemicals to the tank to keep it nice and clean. 

Gray Water Tank

Lastly, we must discuss the gray water tank. This is the wastewater tank that holds water drained from the sinks and showers. While it isn’t as gross as the black water tank, it is still pretty stinky and unpleasant, and we definitely definitely recommend wearing gloves when dealing with it. 

How Large is a Gray Water Tank?

Gray water tanks tend to be bigger than black water tanks. For instance, a small RV might have a 20-gallon black tank and a 30-gallon gray tank. The average gray tank will hold about 40 to 50 gallons, but they do vary quite a lot, so don’t assume this is the size of your gray water tank. 

Just like the other two holding tanks, the size of your gray water tank helps determine how long you can camp without sewer hookups. Since the gray tank tends to fill so quickly, those boondockers who choose to use composting toilets will often convert their black tank to a second gray tank in order to stretch the amount of time they are able to dry camp. 

Using a Gray Water Tank

Using your gray water tank properly is even easier than using the black tank properly. It’s perfectly okay to leave the gray tank open most of the time, and it tends to stay pretty smell- and clog-free all on its own. 

The only thing you really need to know in order to ensure your gray tank stays in tip-top shape is that you shouldn’t put any kind of food down the drain. This includes coffee grounds and semi-solid foods like pudding. These things can clog the tank, and even if they don’t do that, they could definitely make it stink after a while. 

Dumping a Gray Water Tank

The process of dumping a gray tank is almost identical to the process of dumping a black tank. The difference? You’ll pull the gray tank valve rather than the black tank valve.

If you are dumping both the black and gray water tanks—and why wouldn’t you?—we recommend dumping the black tank first, closing its valve (don’t ever skip this step!), and then dumping gray. This allows the less dirty gray water to clean the hose a bit before you remove it from the pipe. (You’ll probably still want to rinse the hose through with some clean water afterward.)

Cleaning a Gray Water Tank

You can clean a gray tank by using the black tank cleaning method above. That said, if you don’t have a way to backflush the gray tank, it is also possible to get it relatively clean by dumping it, filling it completely with bleach water (¼ cup of bleach to every 16 gallons of water), and dumping it again. 

Either way, when you’re finished cleaning the tank, be sure to add chemicals to keep it fresh for as long as possible. 

Dumping RV water tanks
Image source: Mandruss

Basic Water Tank Tips

The information above should be enough to get you started. That said, there are a few more things you might want to know about your holding tanks, especially if you run into problems over time.  The information below will help you address the most common questions and concerns RVers have about their RV holding tanks. 

Winterizing RV Holding Tanks

Before winter temperatures hit, it is incredibly important to winterize your RV’s water system. This includes (but isn’t limited to) the holding tanks. Fortunately, winterizing the holding tanks really only requires that you empty them completely. You might also choose to put some RV antifreeze in the black and gray tanks for extra measure. 

Unclogging Wastewater Tanks

Finding yourself with a clog in your waste tank is super frustrating. In this case, there are a few different remedies you can try:

Boiling water — Sometimes boiling water flushed down the toilet or poured down a drain will break up clogs, making it possible to dump your tanks. 

Back flushing — The backflushing we mentioned before might also do the trick.

Ice — This one may or may not do much, but it’s worth a try. Simply pour a bag of ice into the black tank and go for a drive to slosh everything around and (hopefully) break up the clog. 

Get hands-on — If nothing else works, you might have to go in with a plumbing snake. Be prepared for a gross procedure, and have a partner close at hand. 

Dealing with Smelly Wastewater Tanks

Another common wastewater tank problem? Smells. Fortunately, a good cleaning will get rid of most smells, but if that doesn’t do it, there are a couple of things you can check. 

Check down the vent pipe — Every black tank has a vent pipe on top of the RV. A clogged vent can cause smells to come into the RV. Locate your vent pipe, stick a water hose down it, and turn the water on. Any debris in the pipe will be washed down, allowing stinky smells to properly escape through the vent once again. 

Replace the toilet seals — The seals on RV toilets are made to hold a bit of water in the bowl. This keeps bad smells from coming up. Unfortunately, the seals go bad over time. If your toilet bowl is no longer holding water, replace your seals. That will almost certainly solve your stink issues. 

Repairing RV Holding Tanks

Occasionally, an RV holding tank will develop a crack. This leads to leaking, and sometimes these leaks are really stinky. In many cases, the best course of action is to replace the tank entirely. That said, this is a complicated and somewhat expensive project. If you want to try something else first, we highly recommend Plasti-Mend, a product that chemically welds plastics back together and works nicely on RV holding tanks. 

There you have it, folks. Everything you need to know about RV holding tanks. With this information you can head out on your RV adventures confidently, knowing you have the ability to use, dump, and clean your tanks effectively. 

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