From Goodyear:

Ideally, a vehicle in storage should be placed on blocks to remove all weight from the tires. If the vehicle cannot be put on blocks, follow these steps for tire protection:
· Completely unload the vehicle so that minimum weight will be placed on the tires
· Inflate tires to recommended operating pressure plus 25%. Ensure that the rim manufacturer’s inflation capacity is not exceeded
· Be sure the storage surface is firm, clean, well drained and reasonably level
· Avoid moving the vehicle during extremely cold weather
· Move the vehicle at least every three months to prevent ozone cracking in the tire bulge area, as well as “flat-spotting” from the prolonged strain of sidewall and tread deflection
· Adjust inflation before putting the vehicle back into service
http://www.goodyear.com/rv/pdf/rvbrochure.pdf
---------------------------------------------