Onan propane generator operation at high altitude

As with any other gas appliance it is starving for air......way to fuel rich.

Other than coming down in elevation not much one can do unless you have smaller carb jets with you and can change them out

Onan does have 'high altitude jet' for some models.
You would need to look up model specific manual/parts for optional jet

Norcold/Dometic/Atwood/Suburban have disclaimer about proper functioning above 5500' (Although many of their propane systems function quite very up to 7000' or higher)