3M makes paint protection film in various thicknesses that would work perfectly for minimizing or eliminating road debris dents, chips and such to these lower areas of our vans. Were I adding this material I'd wrap the entire rocker panels beginning a few inches up on the doors and continuing down the body and underneath towards the floor no less than 8-10".
I'd wrap the fenders from the rear (nearest the door) to the front (near the bumper) up a few inches continuing into the inside of the wheel opening cutting and trimming so the material wraps around the fender and folds over the formed horizontal edge, pressed into place so the fender becomes almost encapsulated.
Quarter panels would receive the same treatment but I might consider a thin piece of Lexan etc cut to fit inside the wheel well as a sort of a liner, preventing cast off debris from striking the metal surfaces. Then again something like MonstaLiner et al would work perfectly for this too.
Just ideas from a former body shop owner and current van owner.