Best material to park a 33 ft RV on grass area over winter?

If you are trying to protect your tires from moisture, a material that does not absorb water is best. If you are trying to prevent the tires from sinking into the wet grass any material like wood , concrete, metal will work. I used 3/4 nylon 12"x 12" for the front and 12"x 24" for the rear tires, that act as a moisture barrier to preserve the rubber and also distribute the load so the tires don't sink into the grass.