Hello Keith,
There is a valve with a handle on it in the cupboard under the sink. I can never remember which way I turn it until I try it, but I seem to think that I turn it so the handle points to the back of the van, and then when you turn your taps on, it takes the water from the inboard tank not the aquaroll. If it doesn't,turn it the other way. Through the taps is the only way I know of draining it.
Carl.