All 27Ns have a 30 amp rated shore power / generator transfer switch that should automatically switch over to generator about 20 seconds after the generator starts to run.
The first thing to check is the 30 amp breaker inside the Onan 4000. Location?
You take the side cover off the Onan 4000 generator and you will see a generator start switch towards the bottom center. Look on the side of that box the start switch is on and you will see a 30 amp circuit breaker. On my Onan it tends to trip before the 30 A breaker inside the coach trips. PITA.
The second most likely possibility is that the transfer switch or a connection inside of it has failed and is not switching from shore power to generator power. The lowest possibility is the AC connection inside the generator failed or the generator to transfer switch cable failed.
Randy - Manhattan, Kansas
2015 Vista 27N
2016 Honda HR-V