Your tanks are probably heated by a heat duct that vents into the enclosed underbelly. We have traveled in temps as low as 18 F when we had our 2010 30lsbs. (Just upgraded this year to Silverback). I had a remote temp sensor in the underbelly, we used the furnace and the underbelly never got below 45 F during those colder temps.
If you are using electric heaters this will not heat the underbelly, the furnace should be run enough to keep the underbelly above freezing. I would recommend a remote sensor, they can be purchased at Wal Mart for very little that way you can keep an eye on the temps in the tank area.