Okay just an update. I ended up tapping the brain of a experienced live-aboard
near my slip and we hashed it out a bit more using your input (Orin) and what I had in front of me. Given the degree of access and the state of the y-valve (fairly new) and the history
, I opted to disengage the pipe at the uppermost portion of the holding tank
(i.e head to tank pipe). I chose this to simply depressurize the tank and go from there. I knew with certainty that the portion of pipe from the head to the tank was not blocked. Once depressurized I would be able to flush out the tank with fresh water and suck it out with a wet vac hopefully clearing any potential blockages and attempt to pumpout via the y-valve.
I painstakingly unscrewed the input pipe to the tank while holding the wet vac close by. Small bits of black water and air would escape before I fully disengaged the pipe. I did this for hours with a large amount of baby diapers lining the portion of hull
underneath where i was doing this (genius idea from the live-aboard).
In the end I was able to clear out about 7-10 gallons of black water using the wet vac and multiple trips to the marina head. I took note that aire was rushing into the vent line as I was sucking out the last few gallons which gave me hope that this was over with.
The next day I scheduled a pumpout and was able to get the remainder out. at the same time I did the toilet trick by opening it half way and heard air rushing in. I guided a water hose to the head and filled the toilet with the hopes of flushing
her out even more while at pumpout and it was a success.
In the end I believe it was simply an overfill issue
We went on a three day voyage over memorial day and things seemed fine. No smell (after an exhaustive clean) and the head seems to be working fine! I am going to schedule another pump out before this weekend and hope that all's well.
Thx for all the advice!