You might want to review the troubleshooting section (I think it's section 11) in the ONAN manual. The service light "blinks a code" to identify what the system sensed during the failure. There's a fairly extensive list of "code malfunctions" in the manual. If you don't have the manual, you can download it online. Here's one link:
https://www.manualslib.com/manual/82...b.html?page=89