I have a 11.5c 1500 series 1988 Bigfoot. I have experienced 20 degree temps for two weeks with no issues on the fresh water tank. My water tank is inside the TC, as yours probably is. After a couple of weeks, the gray and black dump valves were froze. After they thawed, I put a couple of quarts of RV anti-freeze in the two tanks. No problems since then.
My guess is, if your setup is such you can run your furnace, you will have no problem. You probably have one duct that goes to your holding tanks. It is not needed for your fresh water. Usually, the problem comes in when you are not connected to shore power and run out of battery for the furnace.
Wayne
