Ah, you're referring to the black tank flush system. Since I don't have to winterize I can't say what I would do. But if you can see the fresh water hose up to the sprayer on the tank I would make sure there's no residual water in it.
As far as the toilet and black and gray tanks. I think based on reading here for many years that most people put a little antifreeze into the waste tanks so that it can flow down to the drain valves and protect them. As far as the tanks as a whole just drain them for winter. The residual liquid in the tanks will freeze. But since the tank is drained there won't be enough ice expansion to damage anything.
In the toilet itself most people pour a little antifreeze to cover the blade seal. Though I've also read about people using mineral oil to cover the blade seal because the pink stuff can stain the plastic bowl.
Bypass and drain the water heater. But do not put antifreeze into it.
