On that coach you first want to make sure you have large mud flaps behind the dual tires to catch the rocks where they start. Have them within 6" of the ground and at least as wide as the tires. If there is room, make them wider.
On the road to Alaska many say not to have the rear mud flap on due to the large dips in the road. If you do have one, make sure it is the recommended 5" off the ground to avoid throwing the rocks as it hits the road.