Some of the older 30 amp systems could run 2 ACs, on the 5500 watt generator, one on each breaker. The 20 amp breaker did not go thru the transfer switch.
The ACs were wired to a switch to chose only one while plugged in to 30 amp shore power.
Plugged in, only one can run but you can choose which one. To run both, start the gen while plugged in.
If everything except one AC works while plugged in AND only 1 AC works with generator power, I suspect the transfer switch is not switching.