Please explain further about "not connected to coach". Does the coach normally plug into the genset (manually) or does it have an auto-transfer switch. Or is the wiring from the genset physically disconnected form its normal place?
It's rare but certainly possible that the Onan wiring has an internal short. The code 13 is consistent with that. Onan op guide only recommends that the load be reduced, but that's because Onan will never recommend an operator start digging inside the genset, which would be necessary for further diagnosis. That is considered unsafe unless one has some electrical skills and knowledge of gensets. Without some idea of your skills, I would have to suggest it needs an Onan-qualfied tech to fix it.
You could replace the breaker, but that's a shot in the dark. With zero external load, even a "weak" breaker isn't going to trip. The good news is that the tripping means the fault is somewhere phsycially after the breaker, which narrows it down.
