I haven't done it, but I would be very skeptical of leaving water in either the black or gray tank (be sure you know the difference) when the trailer won't be heated. Can you drain the tanks before you turn off the heat and leave?
By the way, the black and gray tanks will be hard to drain, because the slide valves will freeze shut. This won't hurt them, but you won't be able to open them. A hairdryer will thaw them, though slowly.
Hadn't heard the idea of using the salt solution, but I would be worried about corroding things like the circulating pump's impeller in the toilet. And if the liquid in the toilet freezes, it would bind up the pump, and you might cause damage if you try to run it before it thaws out.
I suppose you could use RV anti-freeze, about 50/50 mix for a Maine winter. It is not expensive, but it does leave you with more liquid to lug out.
Bill
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