I put a 1" thick piece of the pink foam board that I picked up at Home Depot. With the 3200 lb. weight of the camper on it there has been no compression or any damage. Problem with wood, any wood including marine plywood, is that it absorbs water. The pink foam board does not absorb water or rot or mildew or delaminate.
Best part is that it is "light as a feather" as compared to the weight of a piece of 3/4 inch plywood of the same dimensions. Much easier to remove the foam panel and store when the camper is off the truck.
Also comes in a 2" thickness for more height and more insulation of the camper.