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...
I use NOTHING. The higher the unit is, the higher the vertical Cg is. The base of my TC is 3/4" plywood covered with textured aluminum sheet with roll formed steel runners front to back and one across the front and extreme rear.