OK, On your type set up.
The 30 amp on the Onan supplies the complete RV EXCEPT the Rear AC
The Rear AC should be connected to the 20 amp direct. Also may be connected thru a Front rear switch, but still direct to the 20 amp Onan
That is how 5500 Onans are wired on all Motorhomes.
I suspect your 30 and 20 amp wires are reversed at the Genset. Not an uncommon problem, especially if any work had been done on the system.
Start the Genset, turn the 30 amp breaker on the Onan OFF. If you get power to the rest of the RV your breakers are reversed. Doug