Campgrounds near Washington D.C.?

We camped at Greenbelt for 12 days last month and it was very good for $16/day.
No services on sites, but dump station/water fill and hot showers in washrooms,& 5+ miles of hiking trails.
It's a 30 minute walk to the Greenline train (or 5 minute drive) that takes you to downtown Washington for $2.05 senors rate and a 25 minute ride.