My situation is a little different in that with a Class C, once I start driving down the road the inside of the coach will stay warmed. For the first time this year, since I am taking off within the next two weeks, I am not even winterizing, but running an electric heater at low to maintain temps until I roll out of Ohio.
Those winters when I have needed to head north in the winter and the rig had been previously winterized or I needed to drive through a couple of states and freezing ice storms to make it to the warm of FL in January....I would travel with no water in the fresh water tank, my lines full of pink stuff and with RV antifreeze to put in the Black water tanks and a little in the gray tank to protect what little water that didn't get transferred to the bathroom to be dumped. And I took multiple jugs of water for drinking and washing. I wasn't trying to shower in my rig, I just needed to have enough to drink with, flush and float the Black Tank with, and enough to heat up in the microwave to take a sink bath..... I could travel for weeks like that (borrowing showers at folks houses or at campgrounds....)
Hope that helps. Since the fresh water tank is empty, once I make it to warmer climates all I needed to do was either/or fill my fresh water tank and start using water. It takes very little water to clear what little RV antifreeze in the lines. Soon I have plenty of water for bathing and washing dishes though I keep drinking my jug water for a while until I figure that the tank and the lines are good to go. If I hook up to city water (which I only started doing about two years ago) the principle is the same, the pressure of the water will clear the lines for you.....just remember to let the faucets run until the water clears before using the water...
My fresh water tank in protected from the elements as it is inside under the day bed. My Gray tank and Black Tank are both pretty exposed to the underside and outside temperatures. I figure that they need a little Antifreeze protection regardless....I can only imagine the temperature effect from the wind flowing past on an icy trip on I-75.....