Dangerous way to winterize, Alex...
'Cuz the holding tank isn't the only part of the system that needs winterizing. Water expands when it freezes, so any water left in the intake line, pump, connection to the bowl, or any part of the plumbing that freezes can result in burst hose connections or cracked bowl or pump. What's more, pouring antifreeze down the pumpout doesn't guarantee that enough of it will get into the tank...if there are any bends or low spots in the pumpout line, that's where the antifreeze will sit. Pouring antifreeze into the bowl doesn't do much good either...'cuz nothing poured into the bowl is recirculated through the intake. The ONLY right way to winterize: close the intake seacock, remove the intake hose and stick it into a jug of antifreeze. Pump the whole jug through the system into the tank. Automotive antifreeze should never be used in any toilet, so if you use only non-toxic potable antifreeze--the "pink stuff"--it shouldn't damage in anything your pump. If it does, what y'all really need is a decent quality toilet. It'll cost you more to buy, but a lot less to maintain 'cuz you won't be buying parts every time you turn around, or--as many Jabsco owners do--replacing the whole pump every year.