We just got back from doing part of your itinerary, although we were in a rental car and tent camped. We spent about the same amount of time as you will, realizing we had to prioritize and miss many places along the way. But it was either go and see what we could or not go at all. We particularly paid attention to the days and time of day we were driving certain stretches of highway. For instance we left Cape Cod and drove north through Boston all the way to Bar Harbor in one day. But it was a Sunday and traffic flowed well the whole way. I would imagine we would have spend four or five or more hours for this same trip if we took it on a weekday. Being stuck in an RV for hours during rush hour is no way to spend a vacation, so plan carefully.
Some places aren't especially RV friendly. Newport RI, for example, might be a challenge to find a place to park your RV. We took commuter rail into Boston from the Kingston station. If you are even allowed to take your RV in, you might have to pay for two spaces with the length of your rig.
I would spend your entire days at Mystic and environs and then head to Newport for a whole day there, rather than the back and forth to Newport that you have planned.
At Acadia, make sure you rent bikes and spend a day exploring the carriage roads. Probably the best day of our 17 day trip out there. There are many easy roads, no cars, and great scenery.
We took the Kancamagus Highway instead of going further north as you have planned. Lots of hikes along here plus some great swimming holes that the kids will love (except during spring runoff). Just an alternative to save some miles by not going so far north.