We have taken two different trips along the east coast and have thoroughly enjoyed it. The first trip, we took our 13 and 14 year old niece and nephew and we tent camped. The second trip was last summer - 45 days and 4500 miles. It was fabulous.
We spent several days in DC, riding the METRO only to get around. Cherry Hill seems to be the place to stay and was wonderful. We did the Smithsonian for 2 days (Natural History, Space, American Indian were our favorites). We did the most popular monuments - Washington, Lincoln, WWII, Vietnam, Korean War, then Arlington Cemetery. Most of these were done one day with a couple thrown in while we were at the Smithsonian. We did the zoo another day and it took about 5 hours to cover it and actually look at the animals. That was enough.
From there, we went to Baltimore (we stayed with friends here and parked our RV at their house so I can't help you with a campground here) and took in the aquarium and the Inner Harbor area, then on to Hershey, PA (High Meadows Campground).
High Meadows (FHU)is not a destination campground, but does have 3 pools, arcade, laundry and nice bath houses. It has a shuttle service to/from Hershey Park that is great. You can purchase your tickets at the campground and they are cheaper, you get into the park earlier and can go the day before after 6pm until 10 pm.
On our 1st trip, we went to Philadelphia to see the Liberty Bell, but it was in excess of 100 degrees, so we opted to hotel instead of tent camp. (We wimped). Did Constitution Hall, Liberty Bell and did a tour of Betsy Ross, Ben Franklin and several other places that I really don't remember because I was about the pass out from the heat.
Then on to NYC. We stayed at Liberty Harbor and as other have said, it is just a parking lot with water/electric, but it's location can't be beat for seeing the city. We got tickets to a couple of shows, we did the Gray Line tour where you can get on/off whenever you want. The campground is a 4 block walk to the PATH, which for $2.50 will take you to either the World Trade Center or to 33rd St (Times Square). You can get the Gray Line tour at 33rd St or switch over to the subway system and travel anywhere in the area. It takes about 30 minutes to walk to the PATH, catch and train and be in Times Square UNLESS it is during rush hour. We usually waited until about 9:30 or 10:00 am to go into the city, then returned about 7 or 8 pm. Worked for us.
We went to Old Orchard Beach, Maine and stayed at the Ne're Family Campground, which is a small, older campground with very few amenities, but it is within walking distance (3 blocks) from the beach, all the restaurants and shops. It was FHU and had great WIFI as well as a pool. Our kids LOVED it since we were so close. It was also across the street from the LOBSTER POUND! It was also cheap! $35/night (out of season) and $50 (in season) compared to $85/night in the large resort campgrounds that were further away, but some did offer shuttle service to the downtown area.
Then we headed on to Acadia and stayed at Narrows 2. It was fine and a good location to head out and see all the area.
Hope this helps and if you have other questions, you can PM me.