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...
The weight of my camper is supported by the frame of the camper on 3 sides, the front and both sides, no weight is supported by the center/bottom. So I use 3/4" wood and a mat to raise mine.