That's how I would do it if I had a macerator. I assume you have pumped out first, so the amount of waste should be minimal especially if you have rinsed the tank a couple of times with freshwater. Propylene Gycol (aka the pink stuff) is not toxic to fish and breaks down quickly.
You should suck the waste, rinse, and suck the waste out before you add any thing to the system. If the tank is empty then if just air in the tank and lines, you may be able to leave open for winter (depending where the boat is located.
You'll find detailed instructions for winterizing both the fresh water system and the sanitation system including the toilet, holding tank and any overboard discharge pump in the "sticky" post (3rd one) "Winterizing Plumbing" in the Plumbing and Sanitation forum.
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