Cherry Hill is nice, not the best area in the world as far as safety but the park is "secluded" (I use that term loosely). Has shopping close by (Home Depot/Grocery etc.). It is probably your best choice as far as access. "Technically" (and I use that term loosely because I wouldn't want to do it) you could walk to the metro but it would be a haul again through a suspect area. That being said if you have a car the metro is only five mins tops and runs quite often, quick trip and takes you most places you want to go in DC (and some you don't).
If you want to stay a little further away there is Patapsco Valley State Park which would give you access to Baltimore and Annapolis with a little bit of a drive.
I have never been but there is apparently a DOD campground nestled right in Annapolis proper by the Naval Academy that gets bad/mixed reviews as it doesn't appear to be well kept (but again this is just from reading).
If you want to commute there are some really nice Campgrounds in Gettysburg and Harpers Ferry, but these are drives (approx 2 hours) to get into the city.
Hope that helps, there really aren't a lot of choices but if DC is your destination I would recommend Cherry Hill it is the best location-wise.