There is a growing number of class A owners who are of the opinion (me included) that the only thing the rear flap is good for is appearance. Other than appearances, it causes more problems than it prevents.
Your best protection is to have a good set of flaps directly behind the rear duals set so they just clear the ground when your air suspension is down.
There are a bunch of threads on this subject. Search "Rear Flaps"
Dennis & Carol
2003, 4006, 41' DutchStar,
Spartan, ISC 8.3L W/BanksPower - 2013 Honda CRV, BlueOx Baseplate, Aventa Bar & Patriot Brake. And the 04 Bird, Sunshine Car.