The bottom of my camper is made of 1 1/2" wide framing. I bought a few pressure treated 2x4s, ripped them to the additional thickness I needed, and screwed them to the framing on the bottom of the camper. I then painted them flat black to match the existing bottom of the camper. I've always used a rubber bed mat in my trucks so the camper doesn't slide around.
I tried using pink foam when I bought the first truck that needed additional camper height, but it broke down over time, and was an additional thing I needed to to do (and store) when I installed or removed the camper from the truck. That's why I went with the permanent solution of adding the 2x4s.