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Drain it.
Whether I'm in Maine or anywhere else, I usually leave water in mine 'til I know overnight temps may get down to 29-30ºF, then park however I need to drain as much as possible. Had to drain it in TX one cold night just to be safe, and always drain it for the winter when in Maine.
I then pull pink RV anti-freeze (safe for drinking water systems) through the system by putting a hose down into a gallon of it and opening each outlet one by (I have a hot & cold at my outdoor sink and a mixed outlet at my outdoor shower), which automatically turns on my pump and pulls it through all the lines. I run each line until I see pink coming out.
In my experience, if you have a pump connected and water lines from the tank, they will freeze and create problems before the tank will. No need to fill or leave a quantity of antifreeze in the tank itself, in my experience.
Then come warmer weather, I flush the whole system in the same way; pull clean water through everything by opening each outlet and letting the pump pull it through 'til it runs clear.
Then I thoroughly disinfect the entire system with a bleach solution, filling the whole tank and driving it around a bit so it sloshes everywhere. I let that sit overnight, then flush with fresh. Sometimes I'll disinfect it mid-summer too, especially if it's been sitting a bit.
Been doing it like this for years with no problems from freezing.
Hope that helps.
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