A Magic Holding Tank!

When we took possession of Island Princess (IP) we used the head several times during the time we were getting her ready for the first cruise.   When we initially moved the boat into our slip, I tried to pump the holding tank out and after running downtown to get the proper fitting, finally figured out that the tank was empty!  Micky and I had only wee'd a couple of times in the tank before Lee Bass came down for a visit.
I figured that after a 2-day cruise to Bahia Honda with three full-grown adults aboard the tank would be nearly full.  Not wanting to let the boat sit until October with that stuff in the tank, today I tried again to pump the tank.
At Boca Chica we have a fine electric disposal system on the seawall near the office and six or seven portable hand-pumped units.  Rather than move the boat, we usually use the manual tanks.
Since I have never successfully pumped this boat out, i was more than a little concerned when I couldn't draw a vacuum on the tank.  After two sessions of vigorous manual pumping in the heat, I had Micky give it a session.  I heard a faint hissing sound, and when I broke the seal on the pumpout hose it was obvious the pump was working and the fittings did not leak.
The other requirement for a good disposal of waste is that the tank is properly  vented.  If you use toilet paper and allow the tank to be overfilled and waste is  forced out of the vent, it can become clogged.  Accessing the vent in IP required removal of the propane locker. After a good 45-minute excavation, the vent tube was able to be touched.
Sure enough, the vent was clogged with grey disgusto!  Flushing the tubing with a strong stream of water and reconnecting it, I pumped with renewed vigor!  The only fluids generated were my own sweat.
This boat is also equipped for direct discharge overboard via a macerator pump.  I knew that the macerator had its own discharge port and that the valve for it had been left open by the last owner.  I guessed that the valve was allowing the tank to vent rather than pump out, so I had checked the macerator pump for running and then closed it today.
I took the hose and shot water down the deck fitting.  Through the vent I could distinctly hear the water splashing directly into the tank.  That meant that the stream of water entering the tank at the BOTTOM was splashing!  The tank was empty!  Again!  Magic!
My theory is that with the macerator valve open, all the fluids in the tank flowed out while the boat sailed.  Thus an "empty" tank.  Except for the "solids", of course!
I filled the tank through the deck fitting and finally managed to get 15 gallons or so pumped back out.
No more magic!