We don't camp on the beaches here in Massachusetts, but do camp close to them. We enjoy 4 wheeling on those beaches and have seen many campers on Cape Cod beaches. It really looks like a lot of fun, but I'm sure that the sand as well as the moist salt air beats the heck out of those campers. Most of the RV's do look like they have spent a lot of time on the beaches and are used specifically for that. I, personally, would not take my RV onto the beach. Well, unless it was beat up like most of the RV's I see when 4 wheeling.
You don't need and are not required to have 4 wheel drive for your RV here in Mass.. But you are required to be able to take your camper on and off of the beach, practice run, prior to getting your beach permit. You also need to have a bunch of other items, such as tow rope, air pressure gauge, blocking wood to change tires....and so forth. All required for your permit.
-paul