You can't beat this campground for a DC visit. DC hotels are pricey, so getting to the Mall to visit the (mostly free!) museums and monuments is the biggest challenge if you stay outside the city. Cherry Hill Park in Beltsville, MD is a great option. In season, their buses run, but even off season a Metro Bus has a stop right at the campground. We opted to make the very short drive to the College Park Metro (subway/train) and park in the garage. It was only about $5 to park for the day. If you've not been to DC, the Metro trains are among the best in the US. If you get the unlimited pass and put it on your iPhone, it's super easy to get around. 

The campground is well run. The bathrooms/showers are very clean and modern. The grounds are well maintained. The sites are all nice and level, but rather close, as expected given the proximity to DC. The staff is helpful and friendly. Security is excellent. We dined at Mamma Lucia just down the street the first night and we all had very good meals. There's a large grocery store in the same shopping center, and IKEA is also very close. 

The only con is the road noise. It's close to 95 and the trucks are loud in the night. We got used to it after the first night. If it had been summer, we would have left the fan on, but as it was November, that wasn't an option. Even so, it was a minor issue and it wouldn't stop us from staying again. 

Bottom line, for a DC RV trip, it doesn't get any better than Cherry Hill Park. We had a very nice visit at Cherry Hill Park, and it was wonderful visiting all the great DC sites again. Fall and Winter are excellent times to visit to avoid the crowds.