I have heard people say they use foam insulation board under their camper as a riser with a bed mat on top. I have heard others say they use the insulation foam but no rubber liner and have no camper...
Both my truck and my camper are "older". When I went to pick up the camper from the previous owner a few years ago I found that the cabover portion on the Bigfoot would sit right down on the cab of the truck without a 3" riser in the bed of the truck.