If you need to raise the camper as the cab is so much closer to the cabover then there is another solution. Buy some closed cell insulation, blue board, pink board or gray board...each is very dense and will deform some untill the weight is evenly idstributed, but it also gives extra insulation value, a cush for the camper, is lighter than plywood, will not absorb moisture. In the end I actually built a frame of 1x1's sealed them, inserted blueboard and atached the whole assembly to the bottom of my camper. was like that for years. Do not use the white stuff, it hold water and fall apart in little balls.
