Is It OK to Leave the Gray Tank Valve Open On My RV?

If you’ve stayed at campgrounds and RV parks with full hookups, you may have found yourself wondering, “Is it ok to leave the gray tank valve open?”

That’s a great question… with a bit of a controversial answer.

One of the luxuries of staying in a full-hook-up campground is the ability to use water freely for showers, cooking, and washing dishes, as the water runs freely. The sewer connection allows you to easily let water right out of the RV instead of keeping the gray tank closed and having it fill up.

You’d think that leaving your RV’s gray valve open when fully hooked up would be great because you can take long showers without worrying about filling up your gray tank, right? (At least as long as you have enough hot water.)

But many people leave the gray valve closed because they believe that leaving it open will allow sewer smells to come up into their RV from the campground sewer system.

In this post, we’ll show you how to leave your gray tank open without suffering through the experience of sewer odors.

Does Leaving the Gray Tank Valve Open Cause Odors In an RV?

Everyone expects RV black water to stink. But many aren’t aware of how bad the smell of gray water can be. After all, it’s just sink and shower water, right?

Well, when you mix everything that goes down your shower and sink drains (including food particles, toothpaste, body oil, soaps, etc), gray water can quickly become pretty nasty. That’s especially true during long boondocking trips when RVers severely limit water use.

However, since the drains under every sink and shower in an RV are equipped with a p-trap, leaving the gray water valve open shouldn’t allow odors into the RV. The water in the traps blocks odors from entering the RV’s living space.

Long periods without use can allow the water in the p-traps to evaporate, and driving on twisty or bumpy roads can cause it to slosh out. But both of these situations are easily remedied by running a small amount of water down each drain.

However, there is a pipe with a direct connection to the sewer hose that leads to the RV holding tank vents on the roof of the RV.

This can allow air from the sewer system to come up through the roof vents.

If you’ve ever smelled sewer odors in a full hook-up campground, it’s possible that someone’s RV is venting the park’s sewer system out through their roof.

But if you leave your gray tank valve closed, you’ll need to monitor the tank and dump it as needed — every couple of days, or more, depending on how much you focus on conserving water. And that would be the case even if you’re staying at a full hook-up RV park.

It seems a shame to have to do that when you’re hooked up to the RV park sewer system. When we settle into a full hook-up for a while, part of the luxury of being there is to not have to worry so much about water for a change (unlike when we’re boondocking).

We want to do laundry on board, shower without drizzling water, etc. Sometimes it’s just nice to be able to forget about monitoring water usage so closely.

How to Keep Sewer Odors from Coming Up Through the Roof Vents

If you want to keep your gray tank valve open but you want to keep sewer odors from coming up through your roof vents, there’s a very simple solution.

All you need to do is leave a section of your sewer hose hanging off the end of your sewer hose support.

What this does is create a whole-house p-trap in that small section of hose that will hold accumulated water. That prevents odors from coming up out of the RV park’s sewer system and making their way through your sewer hose and up onto the roof (or into the RV interior).

To see this process visually, have a look at our YouTube video demonstrating how we do it:

This trick will enable you to enjoy the full benefits of full hook-up sites without having to constantly monitor and dump the RV gray tank, and with no sewer odors. And as far as “sewer flies” go, we’ve heard a few stories about them, but have never once seen one in 20 years of full-timing with the gray valve open at full-hook-up campsites.

If you don’t have an RV sewer hose support, (or you need a new one), we highly recommend the Valterra Slunky. Ours works great and is still like new after years of use.

If you’re interested in our (awesome) 360 Siphon roof vents, here’s a link, as well as a video tutorial on how to install them.

Can I Leave My RV Black Tank Valve Open When Connected to a Sewer?

Absolutely. Positively. NOT!

The black valve can NEVER be left open while using your RV! Full hook-ups or not, the RV black tank valve MUST be kept closed.

Leaving the black valve open would allow the “liquids” to run out of the tank, leaving the “solids” behind to form into a pile (known as “pyramiding”). That can be a major cause of tank odor, and require professional intervention to remove.

Even though you’re connected to the sewer, you’ll need to monitor and empty your black tank as needed. Luckily for us, we only need to dump our black tank about once every two weeks (two people, 45-gallon black tank).

Although we didn’t have pyramiding, we’d like to send out a special thanks to Mike Roe of HydroCleanAZ for super-cleaning our tanks in the video below.

Do You Leave Your Gray Tank Valve Open?

So, do you typically leave your gray tank valve open or closed when you’re connected to full hook-ups at a campground or RV park? Leave a comment below.

And hopefully, you know the answer to the question, “Can you dump RV gray water on the ground?” If you don’t, be sure to check out our post.

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