Plus 1 for the Yellow Tri-levelers, from Camco.
Rocks don't stick into the large support pattern on the bottom. Although they're a little bulky for storage, they're super easy to clean and lighter than 2x8. However, I keep a few pieces of 2x around for extra lift. If you place a length of 2x8 on top of the Tri-leveler, you get an (almost) infinitely variable ramp. They work well for my single rear wheel truck, you would simply need one for each rear wheel. If you tie a little rope to the handle, it makes retrieving the inner-wheel leveler easier, without crawling under the camper.