A 36 foot 5th wheel is pretty big for a forest service or national park campground. By the time you are done measuring I bet you are at 50 feet.
This does limit where you can stay. For instance call Yellowstone reservations and see if you can find a place. With a 22 foot travel trailer we would have trouble. There are simply a lot of spots you cannot have a chance for. Remember a smaller unit can take that big space that would fit your 36 foot 5th wheel.
Virtually every National Park will have private RV parks outside of the National Park where you have a better chance.
Good luck!