Grey water draining on. 1990 six-Pac camper

On a friend's sunrader, the grey tank valve is on the opposite side of the camper than the black tank valve. They are completely separate. So I'd look all around the camper for another valve.

Inspect the RV’s underneath. You’ll find it.

The two pulls are usually in a vertical line since you use grey to wash ou the hose when you are done. It’s possible the grey handle is broken off, or hard to see. I’m assuming this camper is new to you. I’ve never seen instructions on how to dump in a manual for the RVs I’ve bought.