For the comments about using antifreeze to flush, you can do that, and I get it, but thought I would share with you that we have left home at 4 below, traveled 400 miles and it was still only 17 degrees. We used water for the toilet and there were no issues with it freezing. That year we were not running the furnace during the day, my olive oil solidified, but, eventually thawed out and was useable!
The next year, we ran the furnace at 45 during the day, as I had my houseplants in the trailer. All was fine. But, that has NOTHING to do with flushing and sub comfy temps.
If you feel safer using antifreeze of some sort to flush, by all means do so, just thought I would share our experiences with plain ole cheap water.
Safe rides ALL, may your toilet never be so cold you cannot deal with sitting on it! EWWWW, BRRRRRRRRRRRRR!
On edit, forgot to say, something we never ever do is drive in snow, first for safety issues, second for all that nasty salt and other road stuff that can and does rust the frame. That's our way, just sharing. Again, be careful out there!