Just a thick rubber mat was enough for mine. It was in two pieces, mostly because it would have been too heavy and unwieldy in a single piece. The mat also kept the camper from moving around in the truck bed. I would try to provide continuous support under the camper floor however it is done.
1994 26' Fleetwood Flair, 454 TBI.
1994 30' Monaco Dynasty, 5.9 230 HP Cummins, MD 3060, 1992 Geo Tracker.