I don't know from your profile whether you are old pros at RV travel, so I will assume otherwise. (Appologies if I'm wrong.) DW and I typically take an 8- or 9-week trip each year, and in 2015 did something like your planned trip in reverse, going from Mobile, AL, to Bar Harbor, Maine, hugging the coastline. We checked out Lancaster County, PA, and western Massachusetts along the way.
One thing to keep in mind is that you can suffer from burnout after days of uninterrupted touring, so we try to take a day off and just loaf once in a while.
Most of the places we visited were very well known, but there were a couple of surprises (for us). Mystic Seaport in Mystic, Conn., may be well-known in the northeast but was new to me They restore old sailing ships. I would love to go back again. The Clark Institute of Art in Williamstown in northwest Massachusetts had an amazing art collection that was a complete suprise to me.
For Boston, New York, and Philadelphia, it is possible to find RV camping close enough for a rail-trip into town. We skipped New York (having visited numerous other times) but made day trips into the other two.
I love to visit WWII ships, and so stopped for the USS Alabama in Mobile and the USS North Carolina in Wilmington, NC. (I skipped the USS New Jersey and USS Massachusetts - can't do it all.)
That's the bottom line: there are so many things to see that you can't possibly work them all in, even on the trip that you are about to take. So take your time, and enjoy what is bound to be a great trip.
