There are three campgrounds that I know of that are fairly close to Metro stops:
Cherry Hill, as has been mentioned many times, is by most all accounts quite nice but relatively expensive.
Greenbelt NP, not far from Cherry Hill, is quite inexpensive and pretty quiet but offers no hookups. It's a large campground so sites are very rarely unavailable. I stayed there for a week a couple years ago (over Thanksgiving week) and it worked out very well for me, walking to the Metro station. It did not strike me as a particularly sketchy neighborhood after dark (or during the day).
Lake Fairfax County Park (in Reston, VA) is somewhat in between the others in terms of amenities and cost; there's a water park and partial hookups, and it's also within reasonable walking distance or a quick drive to a Metro station.
