Bar Harbor, ME we really enjoyed make sure to hit a Lobster Pound and have Popovers at Jordan Pond House.
I'd agree that Washington DC is a must see - and Cherry Hill is a great place to stay - not far from the College Park Metro Station.
Niagara Falls and Niagara on the Lake are worth Visiting.
Our Go to Area (only about an hour or so from home for us) is Lancaster County. A few different campgrounds we go to - depending on what parts we want to hit. You can search our blog for particulars on any of the areas we visited.
Wherever you go, it's better in a RV though! Safe Travels.