This is going to sound like a really stupid question but something that may have been overlooked. You say it has 2 black tanks, 2 grey tanks and a galley tank. Is it possible that the tank you are thinking is the "galley" tank is actually your fresh water tank? The reason I am asking this is that you can see the drain from your sink pulmbed in to the 4" line which if you follow it, it must go to one of the 2 grey tanks. From the other thing you mentioned the tech did find a handle and it was open, if it is still open, try filling up your fresh water tank and see if the water starts running out right away. If it does, close the valve and note that is where your drain valve is for the frsh water tank. The other thing that woud be a good way to identify it is that your waste tanks would be black plastic, while your fresh water tank would be white and somewhat transparent.
Hope this helps solve some of the mystery.