Holding Tanks - Grey or Black?

[AMCbmw]

Well my shower wouldn’t drain. And the grey water drain valve is open. So I ran into town and got a plumbers helper (plunger) - nothing! Then on to Draino - nothing! Next bought a wire snake 10’ and it all went in - nothing! Looked in the tank from the drain hose connection and it was empty- and no snake! Then thought maybe the owners book was wrong, stating “ The term black water refers to waste that is flushed down the toilet and is stored in a separate tank call the black tank. Gray water is the waste water from the sinks shower and tub drains and is stored in the gray tank.“
That’s what I’ve thought for the past RVs I’ve owned, and it was true.
So I bailed out the water with a cup and sponge thinking I’d tackle this when I get back home….
Well, then on a whim I drained the black water tank, sprayed it with the Rino tank flush and guess what! Brown water showed up in the shower pan!
Noooo! I turned the shower on, went outside and sure enough clear water was running out from the black drain, not the grey!
2010 X160, wet bath-VERY WET last night.
FYI

[profdant139]

Wow. what a terrible situation. I can't believe the previous owner put up with this and then conveniently forgot to tell you.

The fix will not be easy, but you obviously need to re-route the shower drain discharge to the gray water. The hardest part will be trying to work under the shower pan -- there is no easy way to get under there.

Here is a suggestion: when I had a leaky shower drain on my old T-139, I cut a rectangular access panel out of the woodwork enclosing the shower pan, using an oscillating multi-tool. After repairing the leak, I then covered the hole with a piece of plywood and then stained it to match the existing paneling. Not beautiful, but adequate.

One more caution -- when working way under the shower pan with your arms extended, watch out for sharp staples sticking out here and there. When I say "watch out," I don't mean it literally, since you can't see what you are doing when your arms are groping around in the dark. I actually mean "be careful."

Please don't ask how I know this.

One more thought -- I think that Drano may be too harsh for the plastics used in RV plumbing. I am not sure I am right about this, though.

[chupp21]

I remember reading the 160 dumps both toilet and shower in the same tank because it's a combo bath. That always sounded like a bad solution to me. I suspect rerouting will be difficult. I wonder if there is a way to convert your toilet to a cassette toilet...
Another thought, If you can't reroute the original drain, you could cut a new shower drain in the side of the pan somewhere and run to your grey tank. Then plug the original drain. You don't need to drain into the top of the grey tank, you can tie into the gray water drain pipe anywhere on the tank side of the dump valve, it will naturally fill the grey tank up through the pipe.

[AMCbmw]

I don’t think the previous owner ever used the shower. Only used the trailer a few times. Now that I know the situation, I’ll accommodate rather than tear into a major re-fitting of the plumbing. Thanks for your response.

[chupp21]

One thing you could do that would be easier, is drill a hole it the backside of the drain pan out the back of the trailer and install a fitting to allow it to drain in a bucket outside. Then have a stopper in the original drain to keep it from going in the tank but you can pull to let the excess drain out of the pan after a shower.
Use a fitting like this: https://www.lascofittings.com/products/H436007

[RipVan]

I’m not positive but I’m pretty sure that both my vanity sink and shower in my ‘07 x210, both drain into the black water tank. I’d have to do a bit of experimenting to be sure. I assume it’s that way for the convenience of the routing of the plumbing as both the vanity sink and the shower are both almost directly above the black water holding tank. I have never and an issues with the situation other than the black water tank filling faster than expected.