Any Cummings dealer that I have dealt with has been extremely expensive and not that good on their service.
I would suggest that you find another GMCer who is familiar with servicing the Onan and get them to help you.
I have worked on many GMCers Onans and have solved a lot of problems. I recently replaced a capacitor and diode in an Onan control panel and it now works fine.
Your signature line doesn't give your location. With that I might be able to recommend a GMCer in your area that could look at it.
Or, if you are going to go the the GMCMI convention in Elkhart I could look at it there.