After looking for a long time I have finally found reasonably priced indoor storage for my Airstream trailer in Minnesota. It's called S&T Boat Storage and Transfer (see S&T Boat Storage | Lake Minnetonka | Maple Plain, MN
). The cost for my Safari 25' is $399 for the summer (May 15 - October 1) and ~$700 for the winter (October 1 - May 15). For a full year that's ~$1100, or $92/month for indoor storage, with in-and-out access.
Now I don't have to worry about hail storms, water leaks and floor rot, and sun deterioration of the finish, plastics, and fabrics!