Originally Posted by awwall
However, you may want to still take Lou's advice. I did the same thing he did because of leaks through closed valves. This will let you get your hose on before you open the valves.
While I do not recommend doing this with Hosrt Probes (standard Gray water probes stick way out into the tank and can attract paper and debris), the end of line valve has another "undocumented" feature when boondocking.
Since the gray water tank typically fills a lot faster than the black, some RV'ers (typically NASCAR campers) use the end of line valve to transfer gray water to the black water tank. They wait until the gray is full, and with the end valve closed, open the gray tank valve. This dumps gray water into the manifold.
Now open the black water tank valve. The higher level in the gray tank will try to equalize by rushing into the black tank. After a few "potatoes"; Close the black valve. Up to 5-8 gallons of gray water will whoosh into the black tank. If done quickly enough, very little (if any) black water will enter the manifold.
Dump as normal; just remembering the mast is full of gray water to the end of line valve.