My boat came with a diaphragm pump that tees off the deck pump out and tees into the sink drain. I know it sounds gross.. but... There is a ball valve directly under the sink. If I close that, pump the tank out through the sink drain thru-hull, and then flush lots of water through it isn't bad at all. Then close the holding tank suction valve, slowly vent the valve under the sink and its all done.. whew... This was very useful cruising through Canadian inside passage where there is NO pumpout facilities anywhere. I don't like the overboard option at anchorages either. I tried to wait for the open water sections before I pumped...
Here is what the system looks like:
I added the valve downstream of the pump so I can lock it shut for legal purposes. The waterline is about right at the top of the holding tank relative to everything in the diagram. From the toilet I can also pump directly overboard via a two-way valve, or pump to the tank where I then get the choice of pumpout at the dock or pump overboard at sea. The normal positions of the valves are shut and shot for the red ones, open for the blue. I can pour water or RV antifreeze into the deck fill to clean out the pump.
Note that the user MUST do the steps properly. Notice that if you don't shut the valve under the galley sink and start pumping there is a very good chance you will fill the sink with poo and pee... this is a very important step to take
After pumping you must shut the holding tank suction and pump discharge or risk filling the holding tank with seawater that leaks by the flapper valves in the pump. While this wouldn't sink the boat it is still a very important observation to make.
I have my suspicions that this isn't going to be "approved" by some sailnetters. All I can say is I didn't do it. Wouldn't have done it, but its there and it works... so...