[JRTJH]

Just thinking out loud here, so this may not even be close to the issue:

I wonder if the factory built your trailer with the valves in the correct location and either before it left the factory or at the dealership, testing showed the gray tank valve to be leaking. Rather than open up the belly to do a valve repair, the technician simply cut the exposed plumbing, installed a second valve and signed off on the gray tank as functional ?????

Stranger things have happened. If I were you, I'd first take down 3 or 4 screws from the coroplast near where the gray plumbing exits the underbelly and actually look at the plumbing where it exits the gray tank and verify that there is not a valve "inside the underbelly"....

I'm "sort of hoping" that you won't find a hidden valve, but I also know that these things are built by more than one person and while it is upside down on the assembly line, when the team is "fitting the underbelly plumbing" it's hard to omit one part and that not be noticed by one of the 6 or 8 people doing the work.

As I said, stranger things have happened and finding a "easy to install" valve under the trailer is definitely a possibility. It's a short cut to getting a problem fixed fast, not right...... YMMV