Here I thought it was the other way around... that you don't want to park on pavement that long, because the tires can't "breathe"... so, gravel, dirt or boards over pavement. We're parked on hard-packed dirt with some gravel right now... hmmmm...
If there was a chance that the dirt would settle, and you'd be trying to pull your RV out of a muddy hole come spring, that might make a difference.
From another site - they mention trying to avoid oily surfaces (i.e. asphalt) and wet surfaces (puddles, grass, etc.). One commenter posted the following quotes:
Michelin: "some storage surfaces can cause tires to age faster"
Bridgestone: "Avoid letting tires sit on asphalt for long periods of time because they can absorb damaging oils from it.
Goodyear: "Thoroughly clean your tires with soap and water before storing them to remove any oils that may have accumulated from the road.”