Whatever you do, don’t attempt to drive around DC in a motorhome, or any other vehicle for that matter! Lots of people recommend cherry stone campground in Maryland. I’ve stayed there twice, and found it to be a very nicely run campground. They cater to RV’rs who are visiting DC, and Have a bus stop that takes you to the metro, which you can also drive to.
The one downside is it is definitely located near highways, evidenced by the constant drone of cars and trucks. Not enough to turn you off to the place, but you will definitely hear it.
DC is a great place to visit. Take the kids to the International Spy Museum in addition to the Smithsonian and the National Zoo.
