I built a set of PVC board leveling planks last week. The PVC is nice because its light and very durable. When they get dirty, just hose them down.
I cut one 12' piece down to three different sizes and made them interlocking so that you could level your wheels anywhere from 3/4 inch to 2 1/4 inches. I used them last weekend and they work great. I need only one board to get level on my parking pad at home, but needed all three on the campground last weekend. I may add another board to the mix to give me more lift if need be.
I am working on a custom bracket system that mounts to the top joists in the storage compartment so that I can slide them in and still have room on the floor below.