If you are into football, Normandy Farms Campground is in Foxboro, Mass, and is about 1 mile from the Pat's stadium. It's one of the highest rated campgrounds in Massachusetts.
But Eastern Mass isn't very RV friendly, unfortunately - that said, the Cape in the fall can be beautiful, and not crowded, but campgrounds close usually right around Columbus day weekend. (all up and down the coast Maine to RI).
I'd recommend Cape Cod Camp Resort (I think it's in Falmouth). Also stayed at Atlantic Oaks, which is about 20 min. from P-Town at the very end of the cape. And on the bike path. Again, check closing dates. And bring heater and warm jackets!
In Gettysburg, we just stayed at Artillery Ridge, and were very happy with that. Artillery Ridge has horseback riding, and is very close to the military visitor center. Bike path nearby also.
Cherry Hill is the campground for Washington.
If you are going through NY, we have stayed at the KOA in Herkimer, NY, and experienced Herkimer Diamond mining, before heading up to Lake George, NY which is also a great place to visit. I recommend Lake George RV Park also, like someone else did. I think there's a bike path connected to it.
NH - I've heard that Coldsprings is a nice resort,but haven't been there. There is also an adult only campground near Coldsprings, i've been told.
Maine - Bailey's has been raved about - haven't been there. It's on the ocean. Point Sebago is nice, on lake. Both are family friendly campgrounds.
Rhode Island -Fishermans State Park is close to the ferry to Block Island, which is a nice place to go.
Hope you have fun planning the trip! I've always wanted to go cross country - but from Massachusetts west!