There are military FamCamps at Quantico, Va which is rustic; Ft Meade, MD which is very nice; Andrews AFB - I have not been there and US Naval Station, Annapolis, MD - very small and primitive.
None are close to the metro but I have driven to a metro station, parked there and road into DC with great ease. Quantico has a train station from the town into Penn Station, DC. Odenton near to Ft Meade has a train station into Penn Station where you can get the metro. Of the famcamps I have been in FT Meade is the best. Full hook-up on all 50 sites (30 or 50 amp svc) pull through or back in. FT Meade phone number is 301-677-6196. If you stay at FT Meade I recommend driving into the New Carrollton Metro station parking the TOW BEAST and ride the metro into DC.
Here is the DC Metro web page
http://www.wmata.com/metrorail/systemmap.cfm
Since your here in July I strongly recommend seeing the USMC Silent Drill Team and Drum and Bugle Corps at Arlington Cemetery at 6PM on the grass and the performance at 8PM Friday night at 8th and I St. Marine Barracks. Reservations are recommended for the Friday show. If you are interested PM me and I will chase it down.
Cheers and Merry Christmas.