Found The Leak!
It's hard to believe. But after taking apart the A/C and doing careful leak testing, the mystery leak has finally been found. And it was not in the A/C as we all had guessed. It was in the forward Fantastic Vent.
Take a look at the picture. As best we can tell, a screw which mounted the vent was overtightened, which cracked the plastic flange (edge) of the vent. The screw worked loose later, which allowed the plastic flange to lift off the roof a tiny bit. As a result, the hairline crack and the loose screw created a "T" shaped gap along the flange (as viewed from the side). This was just enough for heavy rain to seep in, and run through the ceiling to drip out from both vents.
Before the trailer was painted, the crack was very subtle. Once the trailer was painted, the crack was impossible to see. It was found only by applying a spray hose and feeling around for moisture.
The solution is simple: a bit more Vulkem. Problem solved. We'll test it in Missouri, where we are sure to have at least one thunderstorm. Finally ... do I dare say we are done with leaks?
Three days to go before takeoff!