we have a 40' fiver and have found that almost every campground has a site long enough to accommodate us. We have found that not all sites are as they say they are. We can fit into some 35' sites others we can not. Most National Park and Forest Service Campgrounds leave ~10-20% of their sites as first come basis, but from experience we can only fit in a few of those sites so its best to reserve a site. If we are ever in the vicinity of a campground we think we may visit at some point, we try to scope out sites and mark the ones we can fit in for future use.
We have scoped out Apgar Campground in Glacier and they have sites large enough to fit our toyhauler, so they should be able to accommodate you. I just don't recall what those site numbers are.