When we used to begin our winter travel on Jan 2 from Rochester NY we would fill with water in the driveway and dewinterize with the furnace on. The furnace kept the utility bay warm enough and the volume of water in the FW tank was great enough to prevent fast freezing. We drove through some freezing weather and stayed in our son's driveway with temps down to 15 overnight. That coach was an '04 Southwind and before that an '01 Daybreak.
We burned a lot of propane keeping warm but I don't remember ever freezing the water system. I would NOT be happy getting fresh water at FJ. I would stop at a campground along the way however and put some water in the tank for travel purposes. Once you get south of I 40 most campgrounds are open.