Yes, you will occasionally find a park where you cannot get in due to your size. I can think of 3 around here that don't have the slots that big and at least a couple more that only have a very limited number of large slots which are usualy taken. I can also think of other parks close by to those that would accommodate so it's really not a big deal in the grand scheme of things.
Unfortunately, my favorite state park here only has slots 28' long at the largest and there are no other campgrounds close.