Washington DC area campgrounds

Cherry Hill is the closest and most convenient if you plan to use public transportation. That being said, they jam a lot of sites in that park, so there isn't much room to stretch out. If you are just using your camper as a base camp, that isn't a problem. Cherry Hill has FHU sites, a couple pools, a hot tub, I think they just opened a splash pad too. I also believe that they have a Metro bus stop at the entrance to the park that takes you to the College Park Metro station (green line), which takes you straight into DC. You literally don't have to drive anywhere if you don't want too. There is also a grocery store right down the street. Nice Hiker/Biker trails nearby that lead directly into University of Maryland. The wife and I often drive over that direction and take a bike ride into campus and have lunch there.

If you are willing to drive a little further and don't plan to take the metro, I would highly suggest you consider Patapsco State Park. Its about 40 miles north of downtown DC and a beautiful state park. Its a straight shot down Rt 29 or I95 or I295 to get to the heart of DC. Bigger sites, lots of trees, more relaxed vibe. We like to camp at Patapsco, its a very nice state park.