Cherry Hill Park in College Park, Maryland is among the better choices just outside of DC and near Metro. IF CHP is booked, consider what other destinations you will visit while in the area, to help with the search for a campground in Maryland or Virginia. Popular times like the Cherry Blossom Festival you may find closest campgrounds booked in advance.
Good Luck and safe travels
We are staying at Pohick bay regional park, which seems to be another recommended campground. Also I have read that bull run regional park is nice and I think there is Faifax county park. I guess it all depends on the features and distance you want to drive. Good luck.
Apparently Cherry Hill is the only DC park to have a bus connection to Metro. Note that this is not a free service but it will avoid any expenses (monetary or emotional) of driving in the area.
Greenbelt Nat Park is located near to Cherry Hill but offers no hook-up services. It does have a dump station. This would be cheapest possible location for access to Metro.
Other parks are Pohick Bay Regional with full hook-ups, along I-95 south of DC and requiring a drive to a Metro parking lot. Problem is that parking lots fill up with commuters. Bull Run Regional is near Manassas, VA with full hook-ups. Again, you need to drive to Metro parking lot.
Metropolitan DC area is too expensive to support many campgrounds. Look at Cherry Hill rates when you investigate them. BUT - just try to find a cheap, convenient hotel room. Your tax dollars support the free museums, zoo and such but everything else is at market price or more.
Cherry Hill is well known and popular, although I've never stayed there. Pohick Bay Regional Park is also popular and I have stayed there. It is lovely, but hard to get into, generally requiring reservations.
We have stayed at Brunswick City Campground owned/operated by the town of Brunswick MD. It a little ways from DC (50 mi?) but is within walking distance to a major Amtrak commuter station where you can catch a ride to Union Station in DC. From there you can walk to many of the downtown DC attractions, or you can connect to the Metro. I can't recall the camping rates, but I seem to remember they were very reasonable. It is on the Potomac River and also adjacent to the C&O canal towpath park, which is great for bike riding. There is also lots of Civil War history nearby including Harper's Ferry and Antietam battlefield. We love the area.
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