I'm a little late to this thread, but my question is:
Did you have the furnace running? All Airstreams have some sort of tank heat, be it forced air or on the 22s, electric. If it were well below zero, I'd consider running the furnace while in transit. I don't think that practice goes against the design or engineering of the furnace, but it may solve your freezing issues.
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 11/91