It isn't necessarily the size of the site that prohibits a longer motorhome. Sometimes it's because there's a barrier such as a tree right in the middle of the back of the site so there's no room to maneuver. Be sure you know how long the motorhome "really" is because a 40-footer may be 41 feet 5 inches. We got into a site once that was supposed to fit our 40-footer, but there was a chain link fence at the back of the site and the extra foot put our front end out in the street. Another size issue might be have to do with the access roads within the campground. If it's a heavily treed area, you might have problems making turns when trees are close to the road or have low overhanging branches. We've been in both situations and it can be harrowing trying to maneuver without damaging anything. We've also been in campgrounds where there was a length limit but we easily fit into the site and maneuvered the roads without any problem. Generally speaking, if they have a length limit, you need to find out why. If an entry in the Trailer Life Campground Directory states "mostly shaded sites" that means there probably are a lot of trees and if there's also a length limit, the trees may be the reason.
ArdraF