For those of you who have a Reflection 323BHS and probably all models with a corner shower for that matter, the black tank flush check valve is located behind the shower. You need to remove the clear plastic handle in the shower (4 screws) and it is there. My check valve is leaking and dealer will send me a new one.

I took a part just about every cupboard panel in the Fifth Wheel before I found it so hopefully you will get this message before doing like me.