Length restricted rv parks

While not necessarily a restriction, finding open sites for the larger rigs can be challenging (expect for those $100/night resorts!). We are now in MI trying to find campsites and there are plenty available for 30-35 footers but finding a site for my 40' coach has not been successful in the areas we would like to visit (think beach). We were doing find until last week and now all the parks are booked for larger RV's and I may have to stay at Wal-Mart for our next stop before moving on.