I would consider TC weight distribution for choosing the foam.
When 8' camper will give relatively even weight spread over 8' bed, the 12' camper, with COG behind the rear axle tend to rock on last few inches of the bed. I had couple of my 2017 F350 bed ribs bent having 12' camper on factory mat, so imagine what that would do to a foam?
What I noticed is that bed ribs end like 2" before bed end and don't have solid support under.
So I add a stripe of 2" plywood at very end of the bed, where bed has frame under, so now my camper has good support at rocking point.