[Faster]

.......How does a pump out work, anyway? I've never even seen it done.

A pump out connection (ideally on deck, of course) is connected to a line that extends into the holding tank (rigid better than bladder!) down to near the bottom of the tank. The pumpouts are really just powerful vacuums that literally suck the waste out.

This is where the bladder becomes a problem... the bladder itself could suck up onto the pumpout fitting and block it unless it's properly designed to prevent this.

Why not add another deck fitting and do away with the inside-the-boat fill/pumpout altogether?