I've done the following in my T12RSTB:
Grand Teton National Park, October & April/May, several weeks
Temps: high teens/low 20's, snow
Rocky Mountain National Park, December, 5 days
Temps: Low of 9, consistent night time lows in mid teens
Yellowstone National Park, October & April, weeks on end
Temps: Mid teens - low 20s, snow
Not once have I had any trouble with water freezing.
I have thickened up the moulding around the door to seal it better and added reflectix on lots of bare interior areas (not visible areas) including under the bed.
I use the trailers furnace and set it to around 60 when I'm in it in the evening.