What is the black water hook up for

[PhilSacchit]

What is the black water hookup for? I hooked the hose up to this and now the outdoor shower doesn’t have any pressure. https://pin.it/2PjfZcZ

[PhilSacchit]

I just purchased this rig used.
I plugged my city water pressure hose to the black water connection on the panel

Now the outdoor shower barely flows when the city water is hooked to the white one, where as before it blasted out. I thought the black connection was the Poop Tank wash out. Which the previous owners left 1/4 full or so.

Any ideas on what that thing is for?

I have the generic owners manual but I could really use actual plumbing diagrams. Any ideas on where to get one? The manuals that the previous owner gave me do not mention that inlet.
Cheers,
Phil