It's not always the site length that limits the rig. Sometimes it's the corners you need to drive getting there or getting out. I have hosted at an NFS campground that is limited to 28 feet, with sites that would accommodate larger rigs, but the road is too narrow and corners too sharp for those rigs. Having driven Going To The Sun Road, I can assure you that you won't get far into Glacier NP in a 40 foot MH before you find yourself unable to proceed as well as unable to turn around. And you won't like trying to back out at all.