This is from Goodyear:
"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"
Source:
http://www.goodyearrvtires.com/tire-storage.aspx
They also state "Usage per year - more frequent usage will result in longer life" AND "Vehicle storage practices (6 months loaded with little or no rotation is not good!)" This is from Goodyear's RV tire replacement guide. Here's the link if you'd like to take a look.
http://www.goodyearrvtires.com/tire-...uidelines.aspx