Washington DC
Gettysburg, PA
Harpers Ferry, WV
Fort Sumter, Baltimore, MD
Springfield MA - Armory Museum
Also in Springfield, the Smith & Wesson Co. gives free factory tours. Very interesting.
Boston,MA/Plymouth, MA
Assateague Island, MD - the wild horses run free
Hershey, PA (chocolate)
Philadelphia PA (Liberty Bell)
So many places, so little time. I'd suggest you pick and choose what interests you the most and go from there. You don't want to be rushed everywhere you go.
We were at Plymouth Rock last year - sort of neat to stand where it all started.
There is a beautiful park area there to walk around and shops. The rock itself is smaller than expected. Someone else told my husband not to waste his time stopping there, but we really enjoyed it. So everyone is different in what they perceive or are interested in.
We had friends in the area who drove us in to Boston. We did the Duck Tour since I can't do a lot of walking. That was a great way to see the city without driving. I would not recommend driving your MH in to the areas where the heavy city traffic is. Use your toad.
Lancaster County PA (Amish)is a tourist trap - and it's way too commercialized for my taste.
I agree, fall is the best time. The fall foliage is beautiful through the Northeast and the weather is good.
We went to MA April 2012 and we were too early. Most things don't open up until May. Hard to find campgrounds.