Dave gave you the start of the instructions for winterizing the fresh water system--which you do need.. But you need the instructions for winterizing the toilet and holding tank:from "Winterizing Plumbing" in the Plumbing and Sanitation forum. There are two steps--prepping to winterize and winterizing:
SANITATION SYSTEM
The sanitation system is the easy job:
Pump out the holding tank, then rinse thoroughly to flush out any sludge. This does not require filling the tank and can even be done with sea water. Here’s how:
Put enough water into the tank via the deck pumpout fitting—because that sends the water into the tank at the bottom to stir up any sludge and hold it in suspension so it can be flushed out-- to cover the bottom to a depth of at least four inches. Pump that out. Repeat…repeat…repeat…till you’re pumping out clean water. Then add water one more time and turn on the macerator to rinse it out along with the overboard discharge plumbing.
Alternatively, you can use a washdown pump to do the whole job. Stick the nozzle into the deck pumpout out fitting and turn on the water. When the depth reaches a few inches, leave the water running and turn on the macerator pump. Let both run till the water runs clean (it may be best to do this offshore outside the “3 mile limit”).
By the way, this should be done at least 2-3x a season, more often if live aboard or use your boat year round and in preparation for any extended layup, not just winter layup
Now you’re ready to winterize the system.
Sea water toilet, manual or electric: Just flushing antifreeze through the bowl will not protect the intake line, the toilet pump or the bowl...it'll only go out the discharge to the tank. To protect the whole system, it's necessary to close the toilet intake through-hull, disconnect inlet hose and stick it in a gallon of non-toxic ("the pink stuff") antifreeze. Pump the whole gallon through the system into the holding tank. Do not reconnect head intake hose to the through-hull.
If your toilet’s flush water intake line is teed into the head sink drain line, you can add the antifreeze by just pouring it down the sink--after you've closed the drain seacock, of course! Pump the head or hold the button down long enough to get the excess fluid out of the system as possible.
I'll be glad to answer any questions.
--Peggie