On the Luxury 5th wheels, there's an option for tank heating pads - but the tanks are in enclosed, heated underbelllies. If your fresh tank is exposed to the outside air, a heating pad may not be enough. There may not be any reasonable way to keep the fresh water from freezing.
You might be able to put some RV antifreeze in your black and gray tanks to keep them from freezing and use water jugs for the bathroom.