Have you checked out parking it at an RV park? Sometimes they have storage or it might be cheap enough to park it in a regular space. They might give you a monthly rate..
Washington Land yacht harbor in Olympia offers parking/storage for Airstreams. Not sure if you have to be a WBCCI member. Be worth joining just for that if you do need to be. Know several people who left their Airstreams there while they went on a cruise. Low cost and a safe place.
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