Ok... let me see if I can write this out again WITHOUT falling asleep this time...
More information gathered... more pondering has occurred... stuff has been ordered. There is no good order to pen this.
The tank is no where near full ever. The water isn't taking to wing. It must be arriving from above... either the pump out hose or the hose from the vacuflush units. I will prioritize this as vacuflush more likely, pump out less... as the water seems to build with no regard for my trips to the pump out.
The raped and left for dead port in the photo is the pump out so even if it's the other port, I'd be crazy to leave it in that condition, so I need to figure a repair for both.
In a past life, I was something called a Level III examiner for nondestructive testing of aerospace components including tests that used fluorescent dyes to locate surface anomalies (cracks, inclusions, etc). So... since my box with thousands of dollars of magna flux materials cannot be located, I've ordered some cheapo fluorescent dye and another UV light. I'll start by introducing dye to the vacuflush tank and pushing it thru. Hopefully that big bright yellow-green smile of a cracked aerospace part will repeat quickly here.
From the factory... no bladder... the tank is molded in to the hull... a fiberglass cover is glassed over it. The tank connections are robust looking (cf below) FRP glassed into that... into which they screw ABS plumbing fittings. Didn't get the details on how the pump out straw was held in place. In the build photo above, it appears they have not drilled and added the fittings yet. The factory felt they were looking at a screwed up repair in my photos but did not speculate on a path forward.
More context for my actual boat/tank showing the connections between tank and the vacuflushes (via that black t-connection) and the y-valve. Not a fan of that y valve btw... going back to the Jabsco I had before the tech decided to give me that one. You can see a third hose all the way to the left... that comes in via PVC plumbing from the starboard head. The starboard head holds vacuum like a champ.. I have no belief I have a leak from that. The port head is slower to build vacuum and will lose some over a week turned off. I'd suspect a minor leak there but as that path goes nowhere near this space, I don't see a reason to think about that.
Current thoughts about moving forward....
I'm think of the following. Please do not hesitate to correct any lame ideas on my part...
Run the fluorescent dye test... vacuflush side first... pump out second. Identify actual locations of issues.
chip away the bondo... I suspect it's a surface layer from a prior half-a$$ed repair.
Remove the ABS fittings... hopefully the half-a$$er didn't use 5200 as a thread sealant.
Screw two new ones in... suggested sealant anyone?
Attach two new hoses and reapply the clamps.
Seek counseling to get over the bottomless pain of my engine room not smelling like a sewer anymore.