There will be times when it works out and times when it doesn’t. I have a forty footer. Some COE sites that were for 34 or 36 foot max I fit in just fine. Usually that means you back in and hang over the end of the site, but there can’t be a tree in the way. You can’t always tell until you get there.
Other times the site indicates it will take a forty footer but there is no way. We went to Yellowstone a couple of years ago. The first campground we fit in the site OK but the next one was a terrible fit. The site was somewhat curved so once we positioned to accommodate our slides our step was out past the edge of the pad. That made the last step a doozie.
The suggestion of RV parks outside the national park might work most of the time. However, Yellowstone is so huge if you’re in one of those campgrounds you’ll spend all of your time driving to what you want to do.
My point is, unlike those who say “no problem, go for it” I don’t believe it will be that easy. You will make many compromises. But, just like airplanes and boats, everything is a compromise. I’m happy with my choice of a forty footer but I can’t always stay where I want.