DH put on a large flap at the back of the truck one year when we were towing the fifth wheel north from Vancouver Island. The flap protected the fifth wheel fine BUT rocks bounced around under the truck. Picture it - hundreds of ricocheting rocks rattling around underneath. Had to have the underside of the truck box repaired.
The body shop doing the repair also showed us teeny tiny little paint chips (pin point size) on the exterior of the truck box paint. Not visible to the naked eye, yet. They advised those would eventually rust and be more visible and the paint would start to peel. Thus, we chose to have the whole box repainted before rust set in.
The pinpoint chips were everywhere! The body shop said the ricocheting rocks must have flew out from underneath and hit the side of the truck. Now we tow naked, other than the factory mud flaps on the rear wheel wells.