You can usually tell which valve goes where by the color of the handle, and also by the size of the pipe leading into the valve.
My Prowler has a black handle on the black tank and a gray handle on the gray tank.
And, the black tank may have a much larger pipe coming in to the valve than the gray water tank.
I also suggest that you add the Valterra
Twist On Gate Valve ($25)...it will make your life so much better!
Here is a photo of my gate valves: