I contacted the factory about this a few years ago. The answer straight from Airstream was not to place ANYTHING on the rear of the trailer as it was not designed for it and that it was a matter of when, not if, the shell would come detached from the frame in what is called rear end separation.
Here is another thread talking about it:
If you have any doubts, call the factory yourself, you will hear the exact same thing I did.
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