The first line of defense is stopping the rocks where they start, behind the tire. It needs to be the full width of the tires, and preferably an inch out to each side. Make them as long as possible but not so long that they hit the ground when you air down. Without air, put them 5" off the ground.
Then the rear mud flap stops anything that does get under the mud flap, and everything picked up by the wind created under the coach as it moves down the road. It needs to be at least 4" off the ground to keep from hitting the ground or creating a wind tunnel effect that throws rocks up at the toad.