I would think it's actually pretty simple - your generator's own BREAKER has tripped, on the generator itself. It will still 'run', but won't put out any 120v power to run the a/c unit...
it 'could' be your ATS, but I would check the breaker first. If the breaker is reset and it still does not provide power to the a/c unit when running, then remove the ATS cover, and while no shore or gen power, tighten ALL connections. This is not that all uncommon, since when the factory first installs the ATS, and the Main Breaker panel connections, they tighten them while 'cold', but when the generator is running, or shore power is in play, the ATS then has 'hot' wires that can create issues if the connection then loosens over time, which can easily happen.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Hiawassee, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )