Should You Keep Your RV Gray Tank Open or Closed?

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Should you keep your RV gray tank open or closed? My honest answer is the same answer I gave one of our subscribers recently: it depends.

I know that sounds like a terrible answer, but it is true.

This is one of those RV questions that creates a lot of debate because people camp differently, tank sizes vary, and the best answer changes depending on whether you are staying for a weekend or camping in one spot for a week or more.

If you are a new RVer, it is easy to hear one person say always leave the gray tank closed and another person say always leave it open. That is exactly why I wanted to write this article.

My goal here is not to make this more complicated than it needs to be. I just want to tell you how I handle it, why I do it that way, and what I think makes the most sense for most RVers.

My Short Answer

If we are camping for a weekend, I keep both the black tank and gray tank closed.

If we are camping in one spot for a week or longer, I usually leave the gray tank open and the black tank closed. Then I close the gray tank a day or two before dump day so I have enough gray water to help flush the sewer hose after dumping the black tank.

That is the simplest version of my answer.

Why I Keep Both Tanks Closed on Short Trips

If I am only camping for a weekend, I want both tanks to stay closed.

Of course, this depends on the size of your tanks, but in most cases your black and gray tanks should be able to hold all the waste water from a normal weekend trip.

That setup makes dump day easier.

When it is time to dump before heading home, both tanks have a good amount in them. That is what you want. A fuller tank dumps better than one with only a little waste water in it.

That is especially true for the black tank. You want enough liquid and waste in there to help everything move out properly when you pull the valve.

But I also like having a fuller gray tank on a short trip because it gives me plenty of soapy water to rinse the sewer hose after I dump the black tank.

For a weekend trip, keeping both tanks closed just makes the most sense to me.

Why I Often Leave the Gray Tank Open on Longer Stays

If we are staying in one spot for a week or more with a full hookup site, I usually leave the gray tank open.

Why?

Because the gray tank fills up fast, especially if you are taking showers, washing dishes, and using the sinks a lot.

On a longer stay, it can get annoying to keep watching the gray tank level and dumping it over and over. Leaving it open lets that water drain continuously.

But I do not leave the black tank open. I keep the black tank closed until it is time to dump. That part is important.

The black tank needs liquid in it. If you leave the black tank open all the time, the liquids can drain away and leave solids behind. That is how you end up with buildup and a mess you do not want.

So for me, a long stay usually looks like this:

• Gray tank open
• Black tank closed
• Then close the gray tank a day or two before dump day

That way, when I dump the black tank, I still have a good amount of gray water saved up to rinse the hose afterward.

Why I Close the Gray Tank Before Dump Day

This is the part some new RVers miss.

If you leave the gray tank open the whole time and then go to dump the black tank, you may not have enough gray water left to do a good rinse of the sewer hose.

That is why I close the gray tank a day or two before I plan to dump.

I want enough gray water built up so that after I dump the black tank, I can open the gray valve and let the shower and sink water flush out what is left in the hose.

That is one of the best reasons to save gray water before dumping.

The Big Mistake People Make When Leaving the Gray Tank Open

If you decide to leave the gray tank open, there is one very important thing you need to do:

Create a trap in the sewer hose.

This matters for two big reasons.

First, the trap helps block sewer smells from the campground sewer system from coming back up toward your RV.

Second, it also helps keep sewer flies from making their way into your rig.

Think of it like the trap under your kitchen sink at home. That low spot holds water, and that water acts like a barrier.

The same idea works with your RV sewer hose.

If the hose runs straight downhill with no low section holding water, you have less protection from odors and pests coming up from the sewer connection.

How to Create a Sewer Hose Trap

If you are leaving the gray tank open, create a low section in the sewer hose so some gray water stays trapped in the hose.

That trapped water is what helps block the sewer smell.

One easy way to do this is to support the hose so part of it dips low enough to hold water before it rises again toward the RV outlet.

After you set the trap, make sure you have a little water in the gray tank before opening the valve so some water flows down and stays in that low section.

In other words, do not just open the gray valve and assume the trap is working. Make sure there is actually water sitting in that low section.

That water is the whole point.

What About the Advice to Always Keep the Gray Tank Closed?

I understand why a lot of RVers say to always keep both tanks closed.

It is the safest blanket advice because it prevents people from leaving the black tank open, and it also makes sure there is always gray water available to rinse the hose on dump day.

And honestly, if you are new to RVing, keeping both tanks closed is not a bad rule to start with.

But I do not think it has to be a hard rule in every situation.

On longer stays, leaving the gray tank open can be very practical as long as:

• your black tank stays closed
• you create a proper trap in the sewer hose
• you close the gray tank before dump day so you can build up rinse water

That is why my answer is still: it depends.

What I Recommend for Most RVers

Here is what I would tell most RVers, especially beginners:

Weekend trip or short stay: keep both tanks closed.

Longer stay with full hookups: black tank closed, gray tank can be left open if you create a trap in the hose and remember to close the gray tank a day or two before dumping.

That gives you the best mix of convenience and good dumping habits.

Final Thoughts

RVing is full of questions like this where people want one perfect answer, but the truth is a lot of it comes down to how you camp, how long you stay, and what works best for your setup.

For me, the gray tank is not an always open or always closed issue.

It depends on the situation.

On short trips, I want both tanks closed so they are nice and full on dump day.

On longer stays, I am fine leaving the gray tank open as long as I set up a proper sewer hose trap and then close the gray tank before dump day to build up rinse water.

That approach has worked well for us.

If you are new to RVing, do not overcomplicate this. Just remember the most important part: keep the black tank closed until it is time to dump. Then use the gray tank to help rinse everything out afterward.

That will keep you out of a lot of trouble.

Image note: Add the sewer hose trap photo here with a caption explaining that the low section of the hose holds water and acts like a P-trap to block sewer odors and sewer flies when the gray tank valve is open.

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Related Reading:

• How Do I Support My RV Sewer Hose?
• What are RV Sewer Hose Weights?
• Best Disposable Gloves for Dumping RV Tanks

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