You will ALWAYS find a larger number of sites that a 30 foot trailer will fit into than a 40 foot trailer. A 20 footer will find even more sites in which it will fit. That doesn't mean you can't find plenty of sites for larger trailers but it will never be equal. You need to decide what size trailer you need/want and then fit the sites to that size.
For what it's worth, I've only been asked to leave ONE campground in over 100,000 mils of travel and THAT was after I was all set up and ready to pay for the site. I was told it was not possible to get a trailer that size into the campground in Rainier national Park :H They had no answer when I told them I had not had any difficulty getting in but HOPPED I could get out as easily.
