Looking for RV park west of Wash DC near train to city

I stayed at Greenbelt NP recently. It's about a half hour (fairly brisk) walk to the College Park metro station. The park itself seemed safe enough to me, and the general neighborhood is not bad. No hookups, but the fees are very, very reasonable at $16 per night. When I was there over Thanksgiving week, the campground was nearly empty; I could not see another camper from my site for all but the first night.

Greenbelt is north or northeast of the city center, but is quite readily accessible from the beltway.