What kind of temperatures are you expecting (highs and lows)? Below freezing all the time? Below freezing just at night?
We camp with night time lows in the low 20's without any issues. Day time highs above freezing.
We do not have any enclosed underbelly. So the floors are cool/cold in the morning. Our HTT looses a lot of heat through the bunk ends. In general all we use to heat the HTT is a 1500 watt electric heater, and electric mattress pads. During the day I will put the heater on the floor in front of the stove, with a small 12V fan plugged in overhead to move a little more air around. It works just fine during the day. At night we put the heater on top of the stove. In the morning the camper if it is cold outside it is often a little chilly inside, especially the floors. I will turn on the propane furnace for a few minutes to warm the place back up.
If it is really cold outside, I will crack open the cabinets have plumbing inside them to help distribute a little extra heat to the pipes.
The big thermal masses of water in the tanks will take a long time to freeze, and the freeze from the top down.
We have not had any issues yet. remember, we have only used our HTT, with night time lows in the low 20's, with above freezing daytime highs.