At home now and took the cover off the Onan 5500 and there is an altitude adjustment that is labelled 0, midpoint, and 5000. It is set at the midpoint currently. I wish I had known and would have tried the 5000 (max) setting while at altitude to report back. I pretty much had gen problems the whole way home despite the lower altitudes with multiple fault codes thrown. Almost something new each time I tried to start it. It had difficulties starting and staying running. Now back at home at 1200 feet, I will see if it runs properly again.
2015 Precept 29UM