Met eireann cold snap warning for this week, tempt to go below freezing.
So I went out to drop the water, I opened the frost valve on the heater, although the tank is inside under the bed (Pilote A class), I decided to empty it also. I have looked several times while under the van, but never found a drain valve, so went about using the fill hose to siphon the water out, trough the big screw on inspection cap and found a drain plug, on a string, in the bottom of the tank, same as you have in the sink. Happy days.
An extra item to remember is, if you put antifreeze in your washer bottle, squirt some up onto the screen, I've seen the little pipes that feed up along to the wiper blades freeze and split.