The consensus on the board, and from Andy and Inland (I believe he previously weighed in on this) is that don't lay a cover ON the Airstream. I can guess what happens:Older units the clear coat gets worn and then the Al Clad layer of the aluminum goes next and you definitely have a mark that will NEVER go away. You cannot even polish it away, because it was the out layer that you polish and the inner lay you can't.
However, for those of us with older units that spent 30+ years riding around on unbalanced running gear, we're leaky, and need some long term protection during the rainy and snowy months, there fore put it under a pole barn or one of those erectable shelters, anchored firmly to the ground.
Here's a good link to just one thread talking about shelters, there are others: