Spacer under camper - manufacturer "approved" solution?

Already have rubber mat, and the 1.5" from the 2x6's I've got in there now are bare minimum. I was thinking about attaching treated 2x4's or 2x6's around the perimeter of the bottom of the camper - just not exactly sure where the main structure is supported. I would consider the foam board idea, but this thing already has pretty variable CG due to the solders and tank locations, so I'm not 100% on using something that might compress asymmetrically.