I have found the opposite to be true. I had a 40 ft DP. They would tell me they don't have anything to fit me.
I call back and change my length to 35ft and presto i have a site. I get there and the site is 60ft long. For years I have done this and never an issue. I used to say my 45 footer was 38, never had a problem.
I talked with some hosts one time and they told me the rule they used was make the site twice as long as someone asked for. This way no one came back and complained. If they didn't have one twice as long, the answer was sorry we don't have anything. This was in Yellowstone Grant campground. I have been there 9 times.