Grey Tank won't drain, doesn't fill.

My Bathroom sink is plumbed into the black tank but my shower and kitchen sink go to the grey. I would guess the bathroom sink would go to the nearest tank.

The first thing to do is find out where each sink and the shower drains. Drain both tanks then see which one has water in it after running water in each sink and the shower. Don't just look at the idiot lights, see which tank drains.

Don't forget to put a few gallons in each tank when you are finished.

Took the dump valve off today and no water would come out if we ran water into the sink.

So does water run out when you run water into Kitchen sink?
So does water run out when you run water into Shower?

Have you drained black tank and then check if water runs out when you run water into Bathroom Sink?

Your model only has 1 black and 1 grey waste tanks. SO that bathroom sink water is going into one of them.
Otherwise........water would be running out from under trailer

I've read on this forum that people have found the disk from the hole saw used to cut the plumbing holes. Found them inside the tank. The worker pulls back the saw after cutting the hole, the disk might fall from the inside of the hole saw into the tank and be difficult to get out. Nobody's gonna know, maybe, he thinks? And leaves it, or just doesn't realize.

My bathroom sink goes to the black water tank, which is fine with me. It allows me to easily add clean water to the bottom of the tank after dumping. Plus, our gray water fills much more quickly than the black, and any diversion is fine with me.

Good luck, this sounds frustrating. I'd be tempted to try a sewer snake from the bottom up...maybe a manual one, so you could feel what you're hitting, or not hitting.

Harvard wrote:
Some, if not a lot of bathroom sinks are plumbed into the black tank.

This is where I'd put my money.

Maybe there is a leak so it is draining as the sink and shower are used.

Some, if not a lot of bathroom sinks are plumbed into the black tank.