Steved,
Here are my spots:
Corning, NY (Western New York): The Corning Glass Museum. Fascinating place. They have parking for RV's. You can easily spend half a day here, and probably more. (We didn't see it all when we visited. )
Lowell, MA (a little northwest of Boston): There is a textile museum there. Actually, it's much more than that. My wife AND I both find it interesting. She likes the fabric history and I like the machines. The museums at Lowell are great. The town was built where a river makes a horseshoe bend so they could run water wheels by diverting the water. This was one of the first big textile centers in New England. We have always been able to find a place to park our truck and long RV. HIGHLY recommended. Allow at least an entire day.
Shelburne, VT (just south of Burlington): There is a park in Shelburne where a rich lady collected things. Things like houses. The highlight is a steamship that was moved a mile or so from Lake Champlain up to the park. Note the word, "ship," not "boat. " This vessel is about 400 feet long! If you go in the fall, be sure to buy some locally grown apples when they're in season.
If you are a woodworker, Lee-Neilsen tools have a very small shop outside of Warren, Maine, which is near the coast about an hour north of Portland. Lee-Neilsen's motto is, "Heirloom Quality Tools," and they earn it. They make planes mostly, but also dovetail saws. I tried one of their planes across some curly maple and it was like running a bar of soap across a block of ice. ... Okay, I exagerate, but it was clearly the best plane I had ever held. If you want one, bring money. Their plane prices run from about $100 to about $500. (I didn't buy that plane I tried, but I should have. )
Freeport, ME (closer to Portland): The home of L. L. Bean. Open 24 hours, 365 days per year. There are no locks on the doors. There are also lots of other good shops in Freeport, including an L. L. Bean outlet store with some really good deals. There's a small cutlery store that sells a really good pairing knife. (I did buy one of those. ) Lots of parking, including for RV's, and you can walk to almost everything you want to see in town.
We went to New England the first time in October to see the fall foliage (the locals call us "Leaf Peepers"), but we were too late. If you want to catch the famous colors on the leaves, go in September. Either month, it will be chilly, if not downright cold. Pack accordingly.
I'll expect you to post the new places you discover after you get back.
Have a great time.
Loren