Hi Relayman,
When you run the furnace, warm air goes through a 2" duct into the underbelly, providing some heat to the holding tanks. If you use the furnace to keep inside temps around 72 (F), the underbelly temperature may be 10-15 degrees warmer than outside temps.
If your rig came with the YETI option, you'll also have heating pads on the tanks, and should have a switch to turn them on. The YETI option will protect you to much lower temperatures. If you don't know whether you have the YETI option, and can't find a switch, you probably don't have it. A call to Heartland with your VIN # can uncover what options you actually have.