What are you trying to flush? Tennis balls?

No, but pretty close.

Also, what do you think about the idea of eliminating the 'Y' valve in
the system and just having direct to holding tank with the deck
pump-out? In a pinch, couldn't one manually pump it overboard from the
deck? Are not the macerator and Y vales the most likely place for
clogs to occur?

Mine goes direct to the holding tank with a manual pump on a y-valve.
This end of the system has been trouble free.

So the Y valve is_before_the holding tank? Isn't that illegal in many
areas?

Why would you go to a holding tank if you're planning to go offshore?

I prefer not to dump raw sewage into the ocean. Otherwise, I would
just crap over the side.

But thanks for the input!