Most gravel pits will have a bunch of trashed conveyor belt laying around. I get it periodically for walking track equipment on roadways that can’t be damaged.
That’s not your answer though just like stacking other heavy materials.
You can use the plywood you have. Either cut it down the middle and secure it to the bed, or get a 2nd sheet and cut to fit around the wheel wells and split down the middle as previously explained.
Foam is another option. Don’t use the bead board you were talking about. Use rigid polystyrene. It comes in 40, 60, and 100psi compressive strength. 40 will do. It will not crush under an evenly applied load.
The biggest issue though is sliding, right? Your camper is slippery on the bottom. You have 2 options. Contain it in a frame or make it grip better