You do not want to leave the black water dump valve open. The liquids will run off, leaving the solids behind.
You should also keep an eye on the dump valves. If they are like most, they are not in a heated space. They will freeze.
The dump valves in my coach are both in the trunk. I can put a small space heater there. It helps, but in extremely cold weather. I doubt it would prevent freezing.
I have used windshield washer fluid to flush the toilet with some success. It most certainly has to be the winter formula.
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