How to empty tanks on a Palomino Solair 267BHSK

It sounds like they have brought the grey tank outlet into the system ahead of the black tank valve. It's not the way I have seen it done usually, which would be both tanks empty into a common outlet with their own valves, but it would still work OK, who knows...maybe even better? Still, I like the idea that the black tank is done, and whatever is left behind in there cant come out anymore, and the grey flushes out the line to the outlet.