Some folks have suggested vapor barrier matts of some type to segregate the tires. The tire guys will have the best recommendation.
What you do not want to do is use anything that could tip up and damage the coach underbelly or dump valves. Examples might be bricks, 12x12 pavers, blocks of wood, or chunks of concrete.
My suggestion would be to use the coach often enough to keep the tires regularly moving. Unfortunately, not always possible due to work and family scheduling. However, a truly worthy objective.
Pat