Onan Generator 4000 Fault 12 - Overvoltage

I bought a used 2011 Forest River Sunseeker 3120ds last year and have been thoroughly enjoying it with my family. On the last road trip, we were using the generator at Lassen Volcanic National Park (6000+ ft elevation) and it died suddenly flashing a code 12. We looked up code 12 in the manual which means overvoltage. Makes sense as we had the AC on and the halftime oven going.

The generator will start up no problem and run fine until the 'click' occurs when the load is applied and then it dies instantly.

It is an Onan Generator 4000 with 28 hours on it. Yes, only 28.

If we shut off all the circuit breakers in the coach the generator will keep running fine. If we shut off the AC, microwave, HW tank etc and turn on the breakers it will still run. But the moment the AC is turned on, or the microwave starts, or any other load the genset dies instantly giving a code 12.

Oil is fine, filter looks clean, turned main circuit breaker on the genset off and then on again to no avail. Checked all RV plugs to makes sure nothing there is 'shorting out' something and couldn't find any problems. If the RV is plugged in to shore power everything works fine and no breakers trip or anything like that.

Any ideas how I can fix the generator problem?

Thanks in advance