If you want to do the Freedom Trail walk, be aware that Boston isn't a fun place to drive with an RV. You might consider Scusset Beach State Reservation south of Plymouth. Beach within walking distance and there are several options to take the train into Boston. We stayed at Shawme Crowell State Forest as Scusset Beach was undergoing renovation this spring. Scusset is also relatively convenient to the ferry terminal if you decide to do the trip to Martha's Vineyard.
Be aware that the USS Constitution is currently in dry dock and you are only allowed to visit a small portion of the forward deck and nothing down below deck.