We all know that we keep the black water tank closed and dump it when it is about 2/3 full. But the grey water tank fills up so quickly from the sinks and shower that it seems like a good idea to leave it open to drain. Well the first reason is that once you have drained the black water tank, you need your grey tank full in order to flush out the sewer hose.
But there are two more reasons you’ll want to keep the grey tank closed. Most sewer systems in RV parks consist of piping coming from each RV space and continuing along to either the septic tank or sewer outlet for the park. Also there will be, depending on the design, a few vent pipes which will rise out of the ground and stand about 10 feet in the air. The purpose of these is to vent the air that is displaced when someone at a site dumps their holding tanks. The air in the lines is being pushed along seeking the path of least resistance to vent. So if you keep your grey tank open and you are situated next or near to someone dumping their tanks, the air that is displaced by the water heads for the nearest opening. Your coach becomes an additional vent! If your grey tank is closed, you are not creating an opening for odors!
The last but not least reason for keeping your Grey tank closed is that creatures live in sewer lines. That’s right critters, bugs, worms etc. and they can find their way up your sewer hose highway into your Grey tank. With the tank 2/3 full even if they are in the hose, they will be flushed back down the drain—where they belong!