Dethleff advantage t6651 where is the hot water drain valve?

[richardww]

I am trying to drain down the hot water system on my Dethleff Advantage t6651.
So far water tank empty, taps open and button out on the frost relief valve.
No water has drained from the heater unit. Is there an in-line drain toggle switch I haven’t found yet and where is it?
My van has an island bed and the frost relief valve is accessed through a small hole in the floor
Am I missing something?