I am assuming you have a rear kitchen model sense you also have an outside kitchen. That valve is for your second grey water tank, for the kitchen. Your first grey water tank is for your bathroom sink and shower. The valve for it would be by the black water valve. You have 4 tanks, one black, two grey an fresh water tank.
Most keep the black and grey next to each other closed while camping. They hook the drain hose up to the kitchen grey water connection and leave that valve open. When ready to leave drain your black tank if it is about half full, flush your black tank if you have a flush system, then drain that grey tank. This allows you to clean your hose out with that grey water tank.
07 Dodge 1500 crew cab with 20" wheels, 08 Forest River Rockwood Signature Ultra-lite 8280SS
"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man."
George Bernard Shaw