Can't be done...and shouldn't be.
As soon as the pumpout or macerator draws the level in the tank down to the top of the discharge fitting at the bottom of the tank, the pumpout or macerator is gonna start sucking air, breaking the prime. So there's no way to get the last inch out of the bottom of the tank. But it does give you a reason to pump enough water through the toilet to stir up any sludge at the bottom so you can pump that out, and dilute that last inch enough to give you a "clean start" after each pumpout. If the pumpout did get it all, the odds of a clog from sludge would increase considerably 'cuz all the pumpout would get is the liquid, leaving the "mud" on the bottom.
