You will always get somebody who will jump in here & say "Ya, but". Ya but nothing. Chances are slim to zero of getting one of those few sites big enough for a 40'er.
Saw a 45' bus conversion in Gros Ventre, Tetons. Lucky him. Got there before the weekend rush & tagged himself the ONLY site in the CG big enough to accommodate him.
Chances are that you will arrive at a NP or SP only to find a class B or a C in the few sites that you could have gotten yourself into. The shorter sites that you don't stand a chance at are empty. Such is the way it goes.
Done some serious trailer cramming more than a few times. Learned to like the challenge, even if my spotter has doubts of her ability to give directions sometimes.