The gray and black are the same tank.
That's why there's only one drain valve.
I was bummed when I bought my 1996 Skyline Week Ender and realized that the shower drained into the black tank.
😞It sure filled up fast.
I had a 1969 Excel TT that didn't have a gray tank. The way the dump valve was set up I bought a cap that I could attach a garden hose to and drain the gray water as we generated it.
Kenny
1996 Jayco 376FB Eagle Series TT
1997 Jayco 246FB Eagle Series TT
1976 Ford F-250 4wd Mercury Marauder 410 - 4V
Regular cabs. The best looking trucks.