Cherry Hill RV Park in DC is the place to stay. You won't have to drive at all to see the sites. They have a shuttle that connects you with the subway right downtown. Parking in DC Mall area is hell and the subway is clean, fast and convenient. You exit about a block from the mall. Plan on at least 2 or 3 days if you want to see it all. And take the shuttle bus that leaves from the mall Smithsonian and takes you out to the Air Museum wing of the Smithsonian near Dulles. Just about every plane that ever flew. I summer in Maine and Acadia is great. Go north to Quoddy Light and you will be standing on the furthest NE point in the US. Very strong tides and whirlpools. There is a great state park campground a few miles west. In New Brunswick stop at the bridge over the reversing falls in St Johns. At Hopewell you can walk on the sea floor at low tide and view the "chocolate rocks". 39 foot tides come back in quickly there and you can watch the tidal "bore" as the tide comes in. Its a standing wave that flows up the bay and up the Chocolate river. There is a wonderful National Park right on the mid coast of NB with many campsites. There are many many historical sites in New England where America as we know it was born. Too many to mention here but get some travel guides. And if you get near BoothBay Me be sure to stay at Shore Hills, the best RV park in the Mid coast and a great quaint little working harbor town.
PS: After Labor Day the crowds are down, the kids are back in school and reservations at choice locations are much easier. And fall in the NE tends to be sunny, dry and cool. By mid Oct the nights can be frosty.