Emptying the grew water tank is basically a repeat of emptying the black water tank. Simply open the grey water valve and the waste water stored in that tank will discharge through the hose into the sewer dump.
I allow plenty of time for this process, and I usually perform numerous other tasks while this is occurring.
I always empty the grey water last so that this water, which is comprised of water from the sinks and shower, is able to rinse out the sewer hose following the discharge of the black water, which is waste solely from the toilet.
Once the grey water tank is empty, I then close the valve and begin disconnecting the sewer hose.
Note: if the sewer hose runs beneath the slide out, then I postpone disconnecting the hose until the slide out has been retracted. I simply don't like crawling around under the slide.