Our former Class had exposed tanks; and since we always tried to get south for a vacation around mid-winter, traveling in freezing temps was our "norm".
We flushed the toilet with antifreeze; and as long as the temps were 20-and-above, if we dumped a little water down the sink drain (dumping out coffee or brushing teeth, for example), we'd add an equal amount of antifreeze. Never had a bit of trouble. One year, we were heading back home and knew we'd have single-digit temps - then we winterized and did not add any liquid of any kind to the system at all (I read somewhere that urine can freeze at 28-degrees).
We also made window inserts from Reflectix foil insulation; and added foam vent inserts. Most nights, our little electric heater kept the MH adequately warm - once in a while, the furnace kicked on if needed.
Rock salt would work in place of antifreeze - but there's an outside chance it won't dissolve and could jam the slide valve partially open. We would watch for sales on antifreeze (Tractor Supply is always a good place for that) and we'd stock up. I always looked at this as "how much would I pay to NOT have to fix a broken/jammed black-tank valve?" The price of a couple of gallons of pink stuff pales in comparison....LOL!