Capitol KOA in Millersville, MD; Duncan's Family Campground in Lothian, MD; Cherry Hill Campground in College Park, MD are all good locations for the DC area.
Cherry Hill would be the closest and most convienient to Metro, but is in a VERY urban setting. It is right off the DC beltway on Rt.1 which is the main drag in College Park, home of the University of MD.
Capitol KOA is a little further away from DC, but is not so Urban. It is located right off of I-97 and MD32 and is centrally located between DC, Baltimore, and Annapolis. Great place if you think you would want to take in those sights too.
Duncans is southeast of DC off MD4. Most rural of the bunch and a very nice campground.
I can't speak personnally about Cherry Hill, but I can on the other two. They are both nice, good ammenities, and offer shuttle bus service to most area attractions including the metro. If you think you might like to include Baltimore and Annapolis in your sightseeing than I would recommend Capitol KOA. If your looking for trees and quiet go to Duncan's.
2004 Dodge Ram 2500 Club Cab 4x4 CTD
2007 Sunline Solaris T-276-SR: Misty Blue/Cherry
2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD