Winterizing any sea water toilet and holding tank
First, pump out and THOROUGHLY rinse out the holding tank...make sure to rinse out any macerator or manual discharge pump too. Run plenty of clean fresh water through the toilet to rinse out the pump and discharge hose. Now you're ready to winterize:
Disconnect the toilet intake line from the thru-hull (if the boat is in the water, it would be a good idea to close the seacock first). Stick it into a jug of non-toxic potable antifreeze--"the pink stuff" (do NOT use automotive antifreeze, it will damage the rubber parts in the toilet) and pump the whole gallon through the system and into the holding tank by pumping the toilet...or if you have an electric toilet that uses sea water, push the flush button. Pump enough times after the jug is empty to move any antifreeze still in the pump and discharge line to the tank. Leave the head intake line disconnected till spring. If the boat is out of the water, leave the seacock(s) open.
That's all there is to it.
If your toilet uses onboard pressurized flush water... After you've added antifreeze to the fresh water system, flush the toilet till you see pink flush water. Add antifreeze to the tank by pouring it down the toilet.