Mysterious grey water in rear shower base.

[Pulltoy]

Is it possible our kitchen grey tank overflowed and somehow flooded the rear shower? No one has used the shower ever. The black/grey tank is empty. At the same time the kitchen sink drain was acting strange. I think we forgot to dump that grey tank and caused the problem. We’ve been camping for 6 years and was thinking, we probably have done just about all the stupid mistakes there are, and now this mystery?….�� This is on a 2017 397th.

[sande005]

How many grey tanks - I'd guess two. One for main bath and laundry, one for kitchen and back bath. If so, the back wasn't emptied, and the shower was lower than the kitchen, so it backed up there first.
If there really is only one (104 gal is a huge single tank!) same idea, perhaps with the front bath drains all slightly higher than the back, or the trailer being slightly nose up at some point.

[Pulltoy]

Actually 3 but the rear tank is a combination black/grey serving only back toilet, vanity,and shower. That tank was empty but we figured the kitchen grey is somehow plumbed in with it possibly via the vent stack. I’m sure what we thought at the time and now your answer confirms it. I’d love to find a drain schematic to fully understand. Thank you for your input.

[traveldawg]

GD plumbing is a bit weird. I'm not sure about the OPs rig, but on mine if any gray tank fills it backs up into the vent pipe and into another available tank. Not sure what happens when the black tank fills.

Mostly when my main gray fills it backs up into the shower drain.