Correct board for raising camper?

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