We have not been with an RV to Cape Cod (yet) but have traveled there several times and it is a beautiful, beautiful area, one of our favorite places in the world. Provincetown is quite cool, a rather artsy and end-of-the-world place and the Seashore is just out there in the middle of the ocean, it seems.
Allow enough time to explore Cape Cod thoroughly.
The towns are all old fishing villages with quaint old homes, stores, etc. There is a handmade pottery-dishware place in Chatham that has lovely things you can buy by the piece or in sets, and if you can take a day to ferry to Nantucket that is also worth the trip. An overnight there would be best, but B&B's and hotels are rather pricey.
Have a good time, and be sure to eat some quahog chowder.