2015 Lance 1172 Too Shallow for 2015 Chevy 3500HD Bed

Ive been using the rubber bed mat and a 3/4 inch 4x8 sheet of foam board from Lowes.They have different thicknesses; I started out with a 1 inch but decided that, though it worked in providing clearance, it was just a bit too tall. Went to the 3/4 and am getting a season out of each piece.
Probably could go longer but it does compress some and want to be on the safe side. My truck is a 2010, camper a Lance 1017. Height to frame rail clearance may be different than yours.

Good luck and see you out there.