I talked to Rockwood Manor today, and it sounds like a winner. Weeknights there's no 20-person limit for cabins, just $10 per person per night for youth groups. Running water, air conditioning, beds... will be nice after 3 days tent camping.
Transportation into DC is a bit of a challenge I guess. He suggested boarding a county bus nearby, going to the metro. He also suggested riding in, but this is supposed to be a day off the bikes, and I don't really want to be herding 12 kids on bikes in the city.
All in all though, it looks like a great place to stay.