A few years ago we left later than normal for our US trek, we left Canada on December 1 and we used hotels until Bowling Green KY, where we de-winterized. You may have to go a bit farther south later in December to be assured of above freezing daytime temps.
We look at the weather forecast, we de-winterize if it's going to be above freezing in the daytime, overnight it's not a problem because we run the furnace and the RV has enclosed water lines and a heating duct pointed towards the enclosed tanks. We haven't tried running the furnace en route during the day.