I was thinking of the same example that Tom cites. Often an RV has a designated "pad" area of some size, but plenty of adjacent room. Maybe a a curb or other block to stop the wheels, but room to overhang. Also, sites may have a rougher or less level space to drive in on and you can utilize some or all of that if yu have a good leveling system (or simply don't care). Many a time I have placed our 40 ft coach on sites described as being 30-35 ft in length. Yu probably won't be able to tell until you see it, though. Sometimes a call to the park will get an informed answer, but you may also get a "by the book" answer, so it's still a crap shoot.
A war story...
We booked a site in a state park described as a 50 ft pull-through. When we get there, we find it has a long entry drive with a 90 degree dog leg onto the site, which was indeed 50 ft. I figure I can maneuver to get in forward, but no practical way to back in or out. But on the other side there is another RV site, with a rig backed up to the edge of our site. I questioned the ranger about the pull-through and he says "You drive out through the other site!" "What happens", says I, "if he doesn't leave before I do?" He seemed surprised that I would ask, but he called the office and discovered the other site would check out the day before we did. He advised them not to rent that site to anybody the next day, so we could get out. I was skeptical if that would be observed, but it stayed open and we drove out the next day.