Glad to hear Rambin' Pines is getting better. I always feel like I have a bit of a connection to them, since my parents lived in that campground for a while. They're also kind of pricey to be annoying people. The pizza delivery service they used to have (not sure if they still do it) was great!
Capital KOA is nice, well situated between Baltimore, DC, and Annapolis. There's some traffic noise during rush hour unfortunately, but that's true of almost all campgrounds in the area to some extent. Greenbelt Park is cheap, no hookups. Cherry Hill park is good, nothing to write home about but well maintained and good as a base to see the city or work in the area (I lived there for two weeks between houses).
We drove through Duncan's Family Campground near Upper Marlboro recently and it seemed fine. Haven't stayed there.
Bull Run Regional Park near Manasses is fine, aside from the noise from the shooting range. Sigh.
We've visited two Maryland state parks - names escaping me at the moment but one is by Ellicott City and the other is near North East MD. Liked both a lot, but getting the 30' trailer into the one by Ellicott City was a bit of a problem, not helped by the fact that it was dark and pouring, so I was completely blind while backing up.
Farther out - friends in WDCU turned us on to Trap Pond State Park in Delaware. Beautiful, and we look forward to going back every year. And of course Shenandoah National Park isn't that far away, which is something everyone should do at least once...
Harper's Ferry KOA is a little tight, but it's great for visiting the town.