If you have the heated underbelly and your furnace is running, your tanks may be being warmed. At least they are not exposed to the direct freezing outside air.
They may be getting residual heat from being in the same area, however in really cold weather they may not be getting enough to prevent freezing. Electric tank heaters are available as an option on many models for this very reason. I believe they are available as a aftermarket item as well.
Your belly and basement is heated when the furnace runs. I think rated to 20*F by Jayco. Provided you only use the furnace and no additional heat.
Earl
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2017 Eagle 293RKDS, factory ordered, lots of mods and upgrades. More than I can list.
2018 Ram 3500 crew cab long bed SRW diesel 6spd Aisin 4wd Tradesman.
B&W Ram puck 5th wheel
Line-X premium
Mopar wheel to wheel steps