I know it's long, I'd ask for folks to read the whole thing to avoid guesses or suggestions that are clearly out of scope. The subject rig is a 21 Jayco 5er which has not exhibited this behavior in 12,000 miles and 70 nights camping.
Suddenly, when I flushed the toilet this week while the A/C is running, distinct black tank odor comes immediately and directly from the A/C ducted vents, even the farthest vent from the bathroom.
Grey Tanks are empty. All sewer connections are closed. All windows are closed. All vents are closed. Batteries are Lithium. All the other guesses folks have (P traps, and boiling batteries, and condensor coils, and air admittance valves and fantastic fans) do not apply here. Black tank odor is getting in to my AC system during tank flushes and being immediately distributed via the ducted vents. (As a test I opened the door and repeated the test with the same result)
If I turn the AC off, no odor seemed to be noticeable during a flush. Likewise, no odor is noticeable until you flush.
I triple rinse-flushed my black tank, and do not notice the smell after doing that. Never have I seen such clear water .... This of course treated the symptom, but I fear the problem will return.
The only PD I was able to do before sending the trailer to off site storage for a few weeks was to get on the roof and quickly visually inspect the black tank vent. It appears well lap sealed, and I could smell black tank gas from it. I did not have a chance to remove the cap (not sure how either).
Note, there is front and rear AC units, both connected to the duct. I frankly did NOT test them independently. The black tank vent is slightly closer to the front A/C unit, but close enough that either unit could be part of the problem.
This thread from another site is the best match to what I am describing, and I am thinking taking the vent caps off and examining whether the stubs are too short and are letting air in to a punctured duct (!) is what I have to do next. I assume the flush disrupts the gas, sends it up the vent where it unfortunately does not completely exit the vent, but rather is introduced directly in to the ductwork:
There are plenty of threads here on irv that are tangential or not matches to the problem described in this thread. Here's to hoping someone here has resolved this very specific problem and can let me know if they think I am on the right track. Being away from the rig makes PD difficult so I like to get armed with ideas so when I have time to diagnose I am prepared.