Because there is no insulation between the bottom of the furnace ductwork and the underbelly, radiant heating of the underbelly occurs when the furnace is running. I don't know about the new models, but on my '04 unit there was also one small hole, about 1 inch in diameter punched thru the duct for actual air flow into the underbelly. I plugged it up. I have camped with temps in the teens using only electric heaters in the living area and not had a problem with the fresh water supply.
I have always felt that the first item to freeze would be the fresh water pickup tube and not the large fresh water tank, especially since it usually exits the tank on the side and is then routed next to the I-beam. With holes in the I-beam for the slide-out, there is nothing to keep freezing air from blowing over the small pickup tube.
If you plan on camping in very cold temps, then don't just add tank heaters, but also insulate or heat the pickup tube as well.
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Larry Day
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK
https://www.picturetrail.com/dayle1
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