Though it’ll quite easily do the job, the 3/4” rubber mat sounds like it might add a hundred pounds or so of unnecessary weight…
What I’ve used since 2005 that works very well is 1”x 4’x8’ dense foam foil faced sheathing board for about $20 bucks at Home Depot….Holds up extremely well long term (does not compress over time) and provides a bit of underside insulation as well…Atop this I use a 1/4” rubber bed mat…Wet with 60 gal FW, two 30# LPG and travel gear camper is about 4,950#…
Without added this inch of spacer the Eagle Cap 995 camper will hit the top of the rear bumper on my 2016 Ram dually…
