Gray water holding tanks can fill up with water quickly from normal tasks such as washing hands, washing dishes, and taking showers. With the gray tank valve open, all this wastewater that would normally accumulate in the gray water holding tank is free to drain directly into the sewer port at your full hook-up campsite.

This means you will no longer need to dump the gray water holding tank. So leaving the gray tank valve open is quite convenient and even somewhat of a luxury.

Diagram of a thin layer of grease on a gray water holding tank sensor causing a misread at the controls in the RV
A thin layer of grease on a gray water holding tank sensor causing a misread at the controls in the RV

Also, with the gray tank valve open, it is less likely that food debris and grease will cling to the sensors in the tank. Thus they will no longer float and close the circuit, causing sensor errors. 

A p trap in the plumbing under the kitchen sink in an RV
A P trap in the plumbing under the kitchen sink in an RV

Keep in mind that most of the drainage plumbing associated with the gray water system has either P traps or HepvO style waterless traps intended to keep sewer gasses out of the RV. 

A p trap added to the RV sewer discharge hose with water in it to create an additional vapor barrier to keep odors away
A P trap added to the sewer discharge hose with water in it to create an additional vapor barrier to keep odors away

It is best practice to put a P trap bend in the sewer discharge hose if you intend on leaving the gray tank valve open. Adding some water in the P trap itself is also recommended; this will further keep sewer gasses away and will work as a barrier against sewer flies. 

One caution: camping in freezing temperatures could lead to freezing and potential breaks in the sewer discharge hose.

Gray water holding tank valve in the closed position during freezing temperatures
Gray water holding tank valve in the closed position during freezing temperatures

So be sure to close the gray tank valve, and drain and disconnect the sewer discharge hose, if you know the temperature will fall below freezing. 

Otherwise, there is no need to keep the gray valve closed when camping at a full hookup campground. 

If you would like to read about whether or not it is a good idea to leave the black water holding tank valve open while camping, check out this article!

Got a question you’d like answered by our certified RV technicians? Submit your question here!