[Bill]

You will be under way for 5-6 hours. Maybe 7 if you are on slow roads. For this amount of time, I can't prove it from experience, but my impression is that you don't have a problem. My reasons?

Water actually freezes quite slowly. If you put a 20-gallon plastic tank of water on your back porch when the temp is zero-degrees, it certainly wouldn't freeze solid in 5-7 hours. A crust of ice might form on the top, depending on how warm (or cold) the water was when you started. But this is not harmful.

The movement of the TM as you drive will continuously mix the water, so if any crust should form, it will immediately be mixed with the other (slightly warmer) water in the tank, and will melt.

The tank is made of a very flexible plastic. If some ice does form, the tank has a lot of give, so it won't break.

Your tank, being inside the RV, is protected from the wind. So temperature counts, but wind chill does not.

Your TM might have been a bit warm (compared to zero degrees) when you closed it up, so the tank has warm(er) air around it. And the water might have been a bit warm (compared to 32 degrees) when you put it in the tank. Either of these would delay freezing for quite a while.

So in the end, I think you are OK. When you get to your destination, turn on the furnace and open the door under the sink so the warmth in the room can get at the tank. But for the few hours you are talking about, I don't think you have a problem.

By the way, if your situation extends to, say, 18 hours with no heat, in 20-below weather, and high wind, then all bets are off. In that case, I am still under the impression that the tank won't burst. It might freeze, which is annoying when you arrive at your destination. But in this case the thing to be concerned about is the water pump - did it freeze?

In any case, your light bulb idea can't hurt.

Bill