black tank valve stuck open

[Tony Turner]

2018 Vista WFE31BE. The black tank valve is stuck open. (I've temporarily solved the nasty-surprise problem with a gate valve on the combined drain.) I can't figure out how to reach (or even see) the valve. I attached the plumbing schematics, and pics of the water bay. I can see the pipe as it leaves the top of the bay, but when I go under the rig, I don't see any pipes.

How do I access the black tank valve?

Thank you!