Lake Fairfax Park
in Reston, VA, might be more convenient to Arlington. But, it does not have sewer hookups (has dump station). Could catch bus to metro rail, according to the web site.
Pohick Bay Regional Park
does have full hookups. It is about ten miles south off I-95.
If you will be visiting people in Arlington, Cherry Hill
(College Park, MD) might be a long commute. But it is probably the best spot for playing tourist with bus service to metro rail stations. We spent a week there doing the tourist thing one summer. I'd probably do it again.