This will apply to coaches with the capability to flush both the gray and black tanks. I don't know what model year WRV made this change.
A)Black tank smell that would gag a maggot no matter how much I flushed the tank.
B)I noticed that when I flushed the gray tank, I could hear the water loudly splashing inside the tank. But when I flushed the black tank, there was little or no sound.
C) I decided to give the black tank a really long flush. With the black dump open and gray dump closed, I proceeded to flush the black tank. Some time later, Sue screams "There's water coming up in the shower".
So, when I flushed the gray tank, only the gray tank got flushed. When the diversion valve was turned to the black position, both tanks were getting water. See "TEST" picture.
We took the valve out intending to replace it. See "WRONG" picture. The tech discovered that with the handle in one position, the flow went out one "out". With the handle in the other position, the flow went out both "outs". Valve was installed incorrectly. Picking up the valve, I would have done the same thing. Looks logical. My owner's manual shows it correctly.
This is rather obscure and you may have the problem and not even know it. Things are smelling much better around here.