I read an article last year, don't recall just where, but it was written by a tire manufacturer regarding rv long term parking. It focused on two points, the "not exercising" issue mentioned earlier on this post and the surface the rv is parked on. The tire rubber contains oils that when the tire is flexed during use is used to keep the rubber "lubricated" and soft. Sitting for extended periods of like six months or a year causes the rubber to dry out, hence dry rot.
The other thing they recommended was to avoid parking for long periods on soil as the moister is bad for the tire. If you have to store it on soil you should put down either planks of wood or pieces of plywood (plastic would also work) to prevent ground contact. The next best thing is asphalt, although my experience in my driveway is after 2 or 3 months you may have slight indentations in the driveway. The best surface they say is to park on concrete. I would add perhaps putting down plastic dinner mats, but that's just me.
My only bad personal experience was when we left our first 5er parked for just over a year in one spot on gravel and the following trip we had two blowouts, the only ones I have ever had. Can't really say there was a direct connection but just sayin'.
2017 Lariat F-350 FX4, CC, SB, 6.7 PSD 4WD
2018 MONTY 3731FL, Res Frig