On my Ultra Supreme the water bays are heated by a duct that comes off the furnace. If you look between the tanks you might see the open end of the duct about middle of the motorhome. The black and gray tank duct comes off the main duct running under the bed. The fresh water duct comes of the main duct running under the kitchen cabinet.
Recently, like 3 weeks ago, I set up incandescent light bulbs in the 2 water bays as the temperature was below freezing at night. Those bulbs kept the bays above 40F even when down to 27F outside. At night I kept the furnace thermostat at 45F.
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