Grey Tank Valve Location on G3500

Does anyone know where the valve for the grey tank is located on the model G3500? The #1 Black tank valve is located in the cabinet below the reel for the electric cord. There is an access panel in the ceiling above the MORryde drawer. Is the grey tank valve also located under this access panel? Any help is greatly appreciated.

Here is the schematic for the 4000. I am sure it is the same on the 3500. The grey tank valve should be between the two grey tanks.