Cherry Hill just outside of Wash DC in Maryland, near College Park, is one of the finest ikn the country. Not cheap though, but worth every penny.
Rocky Gap State Park, in Western Maryland, is a very reasonably priced CG and in a gorgeous natural setting. Reservations are in order there, using the MD state park reservation service on the web. Ask for the "Ash" loop if you have pets and need power. All the other sites have water only. Dump site on the way out of the park.
Gettysburg has many fine campgrounds.