Vaucu-Flush is want you want. Uses fresh water from your holding tank so no sea water smell. Flushes real quick. Pump out at most any marina. But on most boats like what you are looking at; the holding tank is small. So get a macerator, then when your offshore, open the valve, throw a switch, and the holding tank is emptied out. With macerator, the overboard discharge is only cloudy looking water.
Any luck on a 340?