Hi Dougie and welcome.
I’m perhaps not the best to answer this as I don’t drain down (we regularly camp throughout winter and I keep the Exsis slightly warm and dehumidified throughout).
I believe (without looking to check) that one valve drains the hot water tank (as you’ve already identified) and maybe the other one controls the flow of fresh water TO the tank? I hope other members can clarify this point?
The valve on top of the fresh water tank, again as you’ve identified, drains down the tank.
I’m sure someone will be along shortly to answer this properly!
Best wishes,
Barry