Ken....You didn't say where you plan to stay when at Yellowstone. If you haven't already booked it, Fishing Bridge (my favorite) in the center of the park is probably sold out.
Grizzly campground at the west entrance is probably the next best choice. All of this depends on how long your staying. Last time we were there we stayed at Fishing Bridge and drove down to Jackson Hole and the Tetons. We did 500 miles while staying at Fishing Bridge.
Coulter Bay in the Tetons is another nice place, but again, it may be tough to get a spot for a 40' now.