Can't speak for Roosevelt National Forest, but 90% of our camping is in National Forest. We mostly boondock, so its never a concern finding a spot, but have camped in several National Forest Campgrounds. My experience is that if its listed as 35' we can usually get our 40' TH in the site. You'll need to do your due diligence. Whenever we are in close proximity to a Forest Service or National Park Campground we try and scope it out so that if we ever stay we have a good idea on which sites will accommodate our toyhauler.
Sometimes the site requires us to either unload before starting backing into it or when we are partially backed in.
Forest Service Campgrounds can be an enjoyable experience especially after they close for the season. :)