Not as helpful as they seem. If the rear end really drags (wheels or not), the rear wheels lose traction and you are stuck anyway. Saw a guy last summer who came down a short steep grade that leveled out quickly, and he had to get jacked up and boards placed under his rear wheels despite having the "training wheels" on the back. His rear axle tires came right off the ground and he stopped moving.
You can also bend up the back end pretty bad if the place the wheels mount isn't strong enough, and few motorhomes with a long rear overhand have much support way back there..
The other drawback is that the wheels themselves take up a lot of vertical space, so you hit much sooner than if you did not have them.
Just some things to think about...
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL