About the only time I get to Maryland is when my destination is Washington DC and the Smithsonians. The only listing I have for Maryland and the one I use when I go there is:
COSCA REGIONAL PARK, Clinton, Md. See:
http://www.pgparks.com/places/parks/cosca.html
The campground has 25 spaces and although it is not the greatest being a little dated, it is wonderful given the location. Some of the sites can accommodate large RV's and it is almost never full. It is very affordable given the location. Overnight camping $23 with water & elec.,
Seniors pay $13 per night. There is a dump station. It is convenient to get to from the south via Hwy 301 and Hwy 5. To get to the Smithsonians, take Hwy 5 about 4 miles to just inside the loop, and go to the Branch Ave Metro station and take the metro right down to one block from the national mall. We camped here in mid-September and it was 80% empty! Great fun! Enjoy!