5500 watt Onan generator with only one 30 amp breaker?

I would suggest calling Onan for an explanation. The most reasonable explanations I can come up with are either an error in the published specs, or else the output is tied together at the breaker output and you do have a theoretical 60A total output before the breaker trips, 30A from each of two separate windings. Having a two-pole breaker presumably provides better protection in case one winding fails.