Help: Shower filling up with Kitchen sink water. How?

[Wayfinder]

Has anyone had their shower backup with water after running water in the kitchen sink?

The grey tank is empty, for sure. I am being told they have dumped their tanks.

Any way a leak from the water system could fill up the grey tank?

Shower gate valve is open I am told.

Seems like a clog somewhere.

Asking for a friend.

 

UPDATE: I just received word from her that her indicator for the grey tank is NOW showing 100%.  

But question still stands, just in case. Is there any way a grey tank could fill up with a friend water tank/line leak? 

I cannot think of a way, there should be no fresh connections to grey, right?  She travels with no water in fresh tank as well. 

 

Edited Tuesday at 04:24 PM by Wayfinder
update

[Wayfinder]

At least my first suggestion to her was that her tanks were full. But she was sure they were not.  LOL

[Mike and Carol]

If her gray tank is full then water draining from the kitchen sink could cause backed up water to flow back into the bathroom and out of the shower drain.  I don’t know of any connection between the gray tank and the fresh water system.  Gray tank inputs are from the kitchen and the bathroom, one each.  No fresh water inputs.  Mike

[Wayfinder]

7 minutes ago, Mike and Carol said:

  I don’t know of any connection between the gray tank and the fresh water system.  Gray tank inputs are from the kitchen and the bathroom, one each.  No fresh water inputs.  Mike

That's exactly what I was thinking, but I can never know everything. Thanks for the confirmation.

[Ray Kimsey]

Seems the problem was that the gray tank was full but for others: On older trailers (mine for example, a 2020) you might have a manual gray tank valve. Pulled out is open allowing the gray tank to vent as water from kitchen sink flows in.  If it is closed (pushed in) then water could back up into the shower drain if you run a lot of water in the sink. Also, if you leave the valve open while driving down the road, then gray water could slosh up onto the floor from the shower drain. 

[Wayfinder]

Yeah, I've figured out once that I did not open it when my feet were getting wet while at the bathroom sink.  😜

[Townesw]

Are they sure it’s kitchen water? Are they adding water to the black tank using the No Fuss Flush fitting? If they are flushing the black tank and have let it get too full and the shower drain valve is closed the water from the black tank can back up in the black tank vent and flow over into the sink drain and rise up through the shower drain and fill the shower pan and could actually overflow the pan and flood the trailer floor. 
 

Do not rely on the tank level display when diagnosing water level and drainage problems, or any other time really. 

I apologize for the “ands” and the run-on sentence.

Bill

Edited Tuesday at 06:34 PM by Townesw

[Wayfinder]

42 minutes ago, Townesw said:

Are they sure it’s kitchen water? Are they adding water to the black tank using the No Fuss Flush fitting? If they are flushing the black tank and have let it get too full and the shower drain valve is closed the water from the black tank can back up in the black tank vent and flow over into the sink drain and rise up through the shower drain and fill the shower pan and could actually overflow the pan and flood the trailer floor. 
 

Do not rely on the tank level display when diagnosing water level and drainage problems, or any other time really. 
 

Bill

 

All good questions and statements Bill. None should apply to her situation as I understand it. Some how I think she forgot to drain grey tank, or some how left water on trying to get the water to heat up.  

Where is the location of your drains & vents?  My old rig is not like that as I'm aware of.  UPDATE: OH, THIS MUST BE UNDER THE BATHROOM SINK. I have not been under mine yet.

Also, here toilet would overflow when she opened the foot valve if the black tank got that full.  She's never used the flush out yet. I'll be showing her that and other things in a couple weeks (training).

Edited Tuesday at 07:15 PM by Wayfinder

[jd1923]

Is the trailer level? When we park, we level and keep the front 1/2"+ higher. This allows full usage of the FWT and our heads are just higher than our feet in bed.

If the front is a couple inches low, water from the kitchen drain would flow forward. 

[theOrca]

23 hours ago, jd1923 said:

Is the trailer level? When we park, we level and keep the front 1/2"+ higher. This allows full usage of the FWT and our heads are just higher than our feet in bed.

If the front is a couple inches low, water from the kitchen drain would flow forward. 

That also lets you actually use the last 7 gallons of fresh water...

[HDRider]

On 4/7/2026 at 11:30 AM, Mike and Carol said:

Gray tank inputs are from the kitchen and the bathroom, one each.

Are you sure the bathroom sink goes to the gray?

I put soapy cleaner in the toilet, and down the bathroom sink.  Black drained bubbles, but the gray did not.  That caused me to wonder if the bathroom sink does in fact go to the black.

[Mike and Carol]

1 hour ago, HDRider said:

Are you sure the bathroom sink goes to the gray?

I put soapy cleaner in the toilet, and down the bathroom sink.  Black drained bubbles, but the gray did not.  That caused me to wonder if the bathroom sink does in fact go to the black.

In my trailer, yes, I am sure.  Input to the black tank is from the toilet on top.  There is also a vent opening that is on the right side of the black tank.  Except for the dump opening, that’s it.  In my trailer, the bath sink drains down and goes under the shower, joining with the shower drain to empty into the gray tank.  I had an issue once where the bathroom drain pipe unattached itself from the gray tank causing gray water to flow the length of the trailer and out by the back bumper.  I epoxied the drain pipe back onto the gray tank and problem solved.

Your black tank is 15 gallons, gray is 30ish.  That may have had some effect on your bubble test.  I’ve spent plenty of time under the bathroom sink doing work and (in my trailer) there is no connection between the sink and the black tank.  Someone will point out that there is a shared vent connection, but I don’t think that’s what we’re talking about here.  Mike

Edited 4 hours ago by Mike and Carol

[John Dorrer]

1 hour ago, HDRider said:

Are you sure the bathroom sink goes to the gray?

I put soapy cleaner in the toilet, and down the bathroom sink.  Black drained bubbles, but the gray did not.  That caused me to wonder if the bathroom sink does in fact go to the black.

Both sinks and shower drain are plumbed to the gray tank. The toilet drains into the black tank only.

[jd1923]

4 hours ago, HDRider said:

Are you sure the bathroom sink goes to the gray?

I put soapy cleaner in the toilet, and down the bathroom sink.  Black drained bubbles, but the gray did not.  That caused me to wonder if the bathroom sink does in fact go to the black.

Yes, as Mike explained and as John wrote nicely in two short sentences.

Perhaps you did not see bubbles in the gray waste since there was enough gray water to break down the soap, kill the bubbles, which did not occur in your black waste.

For a fact, black and gray waste are two independent systems, except for the venting of sewer gases as @Townesw illustrated above. However, the vents are at a height, a couple of feet taller, where the cross-over of liquids simply could not occur. Gray water would fill the shower floor and overflow into the cabin well before the height of these vents could be reached.

[Townesw]

1 hour ago, jd1923 said:

For a fact, black and gray waste are two independent systems, except for the venting of sewer gases as @Townesw illustrated above. However, the vents are at a height, a couple of feet taller, where the cross-over of liquids simply could not occur. Gray water would fill the shower floor and overflow into the cabin well before the height of these vents could be reached.

Cross-over of water from the black tank into the gray tank through the common vent absolutely can and has occurred. More than one owner has attached a black tank flush hose, turned it on and forgotten about it. When the black tank fills water rises up the black vent and spills over into the gray vent. If the shower drain valve is closed the water comes up through the shower drain. If the shower drain valve is open the gray tank fills and then the water comes up through the shower drain. 

Edited 24 minutes ago by Townesw

[jd1923]

24 minutes ago, Townesw said:

Cross-over of water from the black tank into the gray tank through the common vent absolutely can and has occurred.

I'm having trouble seeing how this could occur. But given this "has occurred" in your experience, please elaborate. 

Wouldn't the toilet bowl overflow first? We had our black tank once right up to the top, tank full and up the neck, 1-2" below the bowl. Had to stop using it until we towed miles to dump tanks. There was no black waste in the gray water.

We also had the grey fill once to where it came up on the shower floor. In this case, we dumped some gray while boondocking since this is legal in in the SW National Forest lands when 500 feet from others and water sources.

In neither case did waste of one kind go up the vent and into the other. I'm failing to see the physics that would cause water to get up to knee height which I believe is the approximate height of the sewer gas vent junction. Let us know. Thanks