My waste tank is purged to the sea or, with an external hose connected,
to the Wall Mart plaza storm sewer like RVers do it, or an external
tank, like my porta pottie from the truck, using air pressure. We
don't need no steenkin' macerator pump, nor no 'nuther pump neither,
except the 12vdc tire pump, which pressurizes the system enough (1 or 2
lbs overpressure) to blow the "ballast" out the marine discharge. I use
a plastic ball valve as a sea cock, and it doesn't leak. Nor does the
joker valve, even with pressure on.

The only time I open the cheap plastic ball valve / sea cock / port /
sea valve is when emptying the holding tank. The only valve, aside
from air vent valves in the system, is the one sea cock ball valve.
There is a tee between the head, through hull / sea cock, and holding
tank pipe.

Two minutes of pumping air into the tank with the air vents closed and
the sea cock open and it all goes out. Then, it farts to let me know
it's done.

Terry K