you can get non toxic antifreeze. We used it in the lavs in our jets over the winters. Kept the thing from freezing. Very expensive to fix and not very fun for the passengers if it was a long flight.
it may take several gallons into each tank to keep it from freezing. Come to think of it we used a little de-ice fluid from our trucks. already warm. it was propyline (Sp?) gylcol instead of ethyline (sp?) glycol.
Michelle TAC MT-0
Looking for a 1962 Flying Cloud