warbler_road
I don't know what the water/AF mix would have to be to keep it from freezing. I didn't worry about that because we were staying in the trailer with the heat on. I just poured a gallon or so in there and flushed with water from the jug.
I think unless you are in EXTREME cold for a prolonged period, or driving a ways in freezing weather, there will be plenty of wiggle room. Also, the worst that can happen is you can't dump cause it's frozen. There's no risk of ice expansion and broken pipes in the sewage system with just a little liquid in it. Hope this helps :-)