There are a number of choices for RV dump stations near Boston. M&L Boston Fuel & Wash is 13 miles from downtown Boston via I-93 North. While you're there, you can get your RV professionally washed and fill it with fresh water. They also have a diesel fuel station, so you can fill up your tank if your vehicle uses that type of fuel.

Beach Rose RV Park is 47 miles from downtown Boston via I-95 North in Salisbury, MA. It's also close to Highway 1 and I-495, making it very accessible. While you're there, you can fill your RV with potable and rinse water and pick up some propane. The dump station is free to use if you stay at the park, but the general public can also use it for a fee.

Salisbury Beach State Reservation is also around 44 miles from the city, just off I-95 North. The dump station is free to use, but the parking fee for the reservation is $14 for Massachusetts residents and $40 for non-residents. Since you have to pay to park there, you may as well stay and enjoy the park. It's right along the coast, so you'll find ample opportunities to swim, boat, or fish in the Atlantic.