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 1 hour ago 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.  😜