Steve, you've got me thinking now about tires. So I did some research. Unfortunately the information that I can find stops just short of really giving definitive answers. If you read between the lines you can actually see the lawyers behind the scenes telling the writers to be careful.
Both Goodyear and Michelin state that prolonged storage on "some storage surfaces may cause tires to age prematurely" However they won't tell you what those surfaces are. One site did say not to store on asphalt or highly reflective surfaces like sand or snow because of the concentrated UV exposure. They both recommend placing something like cardboard, plywood or a plastic card under the tires for prolonged storage.
The do both recommend that storage inside a garage is the best place to store (well duh!)
Of particular interest to me was that they said to reduce the load on the tire while in storage by unloading the RV and or placing the RV on blocks to remove the weight from the tires. This would tend to be a good reason to take weight off your suspension by storing the rig with the hydraulic levelers down, something we always do so that we can open and close the slides as we need to for interior access.
The other interesting thing is that they won't give a guideline for replacement of tires other than with RVs it is more dependant on age rather than mileage, again for most of us here that is a big "no duh!" Only a few sources said that RV tires should easily last 4 to 5 years depending on their treatment and exposure to UV or pollution and stating that they can last much longer under good conditions. However those numbers seem really low to me unless you live in Yuma, AZ and your rig is parked in the direct sunlight on sand.
Here is a link to a PDF file by Michelin
with a lot of good information.