My van has the swivel Thetford toilet that Hymer put in from approximately 2009 to 2012 and the flush system is just an extra water container just like your fresh water canister so they are simple to empty. Personally I wouldn't leave any water in a van, if it freezes and then it expands it will do damage. I suppose the toilet fluid may reduce the freezing point but it is a risk you are taking. As regards the rubber on the slide you can buy a purpose made spray which I use however you can use olive oil. To avoid the rubber seal sticking in the cassette leave the toilet blade in the open position when in winter storage.

To be honest anyone who is going to worry about it should just get on and do it, mine I reckon would take all of minutes, and that would include keeping the drained pink sweet smelling fluid I might even give it to some of you for christmas