Keeping water tank from freezing while towing

Hello everybody.

I am a new TM owner and new to the forum. Your advice on the following question will be much appreciated. Living in Wyoming, it is likely that I will get caught in sudden daytime temperatures well below freezing. How can I keep the water in the drinking water tank from freezing as we roll down the highway? There is no problem when we get to our destination, put the TM up, and turn on the propane heater. But enroute, driving through perhaps subzero weather for 300 miles, there seems to me to be a good chance of freezing up.

One possible solution that occurred to me is to get a 12v Coleman portable car heater which is based on a halogen bulb, plug that into the 12v circuit in the TM, and stick the heater in next to the water tank. While undertow, there would be 12v current in the TM, so this would seem to work.

What do you all think about that procedure, or do you have some better ideas? I am assuming that before the trip, I made sure that the gray water tank was dumped, and perhaps put some rv antifreeze into it. However, I didn't want to drain my fresh water tank, since we wanted water when we set up camp at the end of the day.