So here is the update:
Thank you for the very specific tips and the drawings.
After flushing the tank a few times I filled it to overfull such that water collected in the shower floor.
I waited a while and although the water went down a little at first it eventually held level.
From this I concluded that I had no leaks below the level of the shower. Or at least very small leaks that could not possibly make that smell.
I used a camera to get a shot of the gray tank vent while the tank was overfull and there appears to be no water around it and it looks attached. See picture.
The drain from the slide is easy to get to and also was not leaking.
I can not get to where the drain from the shower goes in but it wasn't dripping water.
As it so happens I just re-sealed the 360 siphon vent on the black tank and while I was up there I pulled on and wiggled the pipe and it seemed attached.
I removed the vacuum break under the shower and inspected it and tested it it seem fine. I put it back on taped up. This had no effect the smell was still intense and would not abate.
I canceled my service appointment and called a mobile tank cleaning guy who has a pressure washer and some drain equipment and he came out and rinsed the tank. The company was Royal Flush of Yuma. Not impressed.
Once he started spraying however the smell seemed to come from everywhere so I so I wonder if the tanks and fittings just aren't up to a very ripe tank and smell just permeates the materials. Normally it only stinks when it is really hot, like over 90 all day and it wasn't very hot when he was cleaning and had not been stinking for a few days.
After he was done I rinsed again a few times and again overfilled with the same results.
My gauge is still stuck at 2/3 and although not the issue at hand I was kind of hoping the tank wash would clean that up.
One remaining disconcerting symptom was the tiny black flies that were in the area around the shower (not sure if they were under the shower) The only other time I have seen these was when when I had the plumbing open. These flies (and the smell) are why I thought there would be a hole in the system somewhere. Again the flies were only there when the coach was 100 degrees. Haven't seen them since.
I am going to order one of these USB Endoscope
for the next time I need to see something hard to get to.
I may try to build my own flushing system. Maybe tie it to the hot water pressure relief valve and put a 360 sprinkler head on a thin hose.
Thank for all the tips. I'll keep trying to find this leak if it really is a leak and not just a systemic design flaw.