We visited Yellowstone several years ago and stayed in two different campgrounds there. We have a 34 ft 5er. There are not a lot of sites that will accommodate large trailers or for that matter large motorhomes. In one site, we almost didn't get the trailer hooked back up to the truck! If you call for reservations, they will help you find a site that your trailer can fit into. I would cheat a little on the side of caution and give them a length slightly longer than my trailer. Just be aware that there aren't a lot of big sites
available.