Here is a link to an RV dealer in the Boston, MA area. You may want to contact them and see if there is a way to rent one on Cape Cod. We did a two week vacation there this summer and had a great time. It looks like they will deliver and set up at a local campground so you may want to fly to Boston, rent a car and drive to the campground and have everything ready for you.
Good luck!
http://www.mcdcorp.com/Rentals.htm