The generator will flash codes suggesting the problem.
Mine went through a period of shutting down on release of the start button because once the generator came online it detected a short in the 120V house system. Another time it was a problem transitioning from idle to light load running speeds, codes for that suggested an ignition or fuel system problem.
Not many RV dealers are licensed Cummins/Onan service points, although a few will do limited work, like changing a spark plug or an oil change.