Since you have already moved the camper back 4" and it still hits the rear bumper that the manufacturer's design leaves something to be desired. Have you investigated the possibility of lowering the rear bumper for clearance? My camper sits on a pick up bed that was Linexed and is a very good non-skid for the camper floor. Your idea of a plywood spacer that has a non-skip finish would be good if you either apply the non-skid to your truck bed or screw the plywood to the bed. I would suggest you look into Torklift Fastgun tiedowns. Far superior to chain set ups.