I'm in the 32-34' max length camp for any boon-docking. There just too many narrow dirt roads with over hanging branches. There are several great primitive campgrounds on the west side of Glacier Park. We checked them out in the toad, but decided not to chance it. The gravel roads in were basically a lane wide with over hanging branches.
The 40-42' diesels are great for RVing, not so good for getting into boon-docking areas. Take a look at the truck camper forum trip reports for some ideas what the "roads" are like.