Question; did you have the air conditioners running on 50 amp shore power before and while
starting the generator?
Your gennie cannot start under that kind of a load- it will pop its own breakers, which sheds the load but allows the gen to continue running.
The service diagram
for your unit indicates the breakers are on the control panel, so it shouldn't be hard to check their condition.
It may be that the auto-transfer switch is bad, because it should automatically change over to the generator (assuming, as I said, that you aren't trying to start the gen under load.)