Junction City, KS is near plenty of great places to explore in an RV, and its location along I-70 makes it a great stopping point on any road trip. Kansas has plenty of RV maintenance resources throughout the state, including the Junction City dump station. You can find free non-potable water at many dump stations as well.
There are a few storage businesses in Kansas that let you use their dump stations at no cost too. If you see an RV storage place, then you can probably dump your wastewater there as well.