Grey & black tank

Update:
For some reason not known to us our black tank gauges a&e reading right again,shows 1/2 full which is right for 4 days but I don't have a explanation why it is ready right know
THANKS all Rich

Just had both my tanks pressure cleaned, $195. Had a problem with incorrect guage reading (black tank always had 3/4 light on), and it never seemed to empty.
Pressure cleaning solved both. It's all done from outside, so no interior mess.
Look around for rv tank techs or tank proctologist.
Very worth the money.

With our coach if there's a question about the black tank fluid level, I can hold the flush valve open (ball valve type) and and using a flashlight visually check the fluid level.

Your tank may not be as empty as you think as there may be stuff piled up in it. Hope not. Your bathroom sink may be draining into the black tank. Ours does.

You may have a pyramid of solids. Open toilet and shine a flashlight down.

Don't dump the black based on your guage. It should start making gurgling type noises when it is really nearing full. If you guage isn't right, you may be dumping when the tank is only partially filled. Neither of our gauges is accurate even though we backflush.

On an earlier class A that we owned, the shower drained into the black holding tank. Yours could also. Do the test mentioned above.

It would be easy to figure out exactly what is going into each tank. Empty all your holding tanks, then simply run water from one faucet only for a couple minutes. Open each valve one at a time and see which one the water comes out.

Pick up a clear section of dump tubing such as THIS. Next time you dump, once the tanks are empty close the gray valve and leave the black open. Turn on the shower and sinks, one at a time, and check if you get water draining from the black tank. This will confirm if you have what would be expected to drain into the gray tank going into the black.

Also consider measuring the capacity of the black tank by filling it with measured bucket then compare it to the OEM capacity. You can also check when the various level indicators come on. Full could be coming on at 3/4 or 2/3.

Our gauge goes from empty to 3/4 full in a couple flushes...

I bet your gauge is not reading right...

I have heard of rigs with a sink that goes to the black tank though...

Might take some investigation...

Nike