Correct board for raising camper?

Part of the compression problem seen is how people size/orient the extruded polystyrene foam (XPS). Sized to just fit under the TC doesn't give a lot of support to the foam edges (my version 1). Placing the sheet(s) across the bed width provides more support.

If you have a light camper, the 15 psi compressive strength of the XPS shown by the original poster will work to raise the TC with little compression. If you have a heavier TC both Owens Corning and Dow make XPS foam in higher compressive strengths of: 25, 30, 40, and 60 psi. The 30-60 psi XPS is used under concrete slabs.

If you need more height than a single sheet of XPS foam provides, they can readily be glued together with a water based contact cementwater based contact cement. I glued 1.5" and 2" sheets of Owens Corning Formular 250 together to get the height I needed.

If you want to prevent movement you can glue together pieces of foam to make a block to prevent the TC from moving side to side or too far forward (permanent or added after loading the TC).

You can also make the foam nonskid by mixing up some epoxy resin (local marine supply store or online), applying it to the foam (where the TC touches) and then lightly broadcast dry, fine, sand onto the wet epoxy (similar to a nonskid surface on a boat). I've successfully used epoxy resin on XPS foam. I don't know whether the cheaper polyester based resins will melt the XPS foam or not.

If you still have issues with crushing the foam, you can always epoxy in a layer of 6" wide fiberglass to the foam in that area to further spread the load. Much lighter than a wood frame.

I cut a 2X4 in half and screwed it to the outer edges of a 4’X8’ sheet of exterior plywood. Then I cut the 1.5” Styrofoam board (like you show) to 3’ 8” X 7’ 8” and glued it inside the plywood frame. I have well over 20,000 miles on this setup and no crushing of the foam board. I believe that the foam board edges will start to compress overtime with the slight tilting of the camper due to wind and sway without a rigid edge.

The foam board supports the center of the plywood.

Just my opinion and it works for me.
LeRoy

I would not use foam, I did and it compressed to the point of giving me almost no cab clearance at the end of an 8000 mile trip.

Kayteg1 wrote:
Composite decking is very slippery stuff.
Once I had it bundled on pickup with 5000lb strap and still few pieces managed to slide out.

I will slip on itself - not so much on a bed-pad. Altho any wood will slip a little if you bounce your truck around no matter what you use, so just make sure your anchors are installed and hooked correctly and you'll be all set. I have comp decking on my camper, and it never moves ...

Composite decking is very slippery stuff.
Once I had it bundled on pickup with 5000lb strap and still few pieces managed to slide out.

If you need to raise you camper to clear rails or cab, cut three/four pieces of one inch composite decking and screw it directly to the bottom of the camper, front, rear contact point, and one or two in the middle, cross-ways on the bed, not length-ways. You can add glue, but that makes it difficult to replace if you ever damage a board. If you need more than one inch, then use 2x4 pressure treated, or 2x6 if you have a long bed camper.

Then, you're camper sits higher and right on the bed-pad, and it never moves. I've done this to both a Fleetwood and Lance. Sixteen years total, mountains, desert, highway, off-highway, whatever ... never has an issue with movement.

PLUS, it gives you ventilation under the camper, so it never sits in water.

I observe that my 6000 lb camper rocks on last few inches of the bed when in the motion.
Doubt that foam would work without being compress badly.
I stick to my wood planks and plywood.

Yes, that foam is 15 psi compressive strength, so it should work fine. But I've never used it under a camper.

That's what I used. And my Outfitter doesn't slip around on it at all.

Get some cheap shelf liner and put that top and bottom if you encounter slip. You just need a gripper, not a thick heavy pad.