I've always believed, but I believe in a lot of stuff that ain't necessarily so, that the issue with freezing is expansion of the ice as it freezes. Thus, in the piping and pumps, and such is where the problem lies. Seems to me that if you have a couple gallons of human water and dish water, and probably some antifreeze in the gray and black tanks, you may have a block of unpleasant ice, but as long as it can expand, it shouldn't hurt the tank. But this too could be one of my hallucinations.
So, the main concern is making sure your pipes, pump, and valves are all filled with antifreeze until the risk of freezing is over. And here in NV it's a sunny 65 today, my battery is 100% charged off the solar, which went from 1/2 on the "meter" in that also has the water tank lights) to full in about a half hour. So life is good.