I agree that it will take a special person to want a used builtin generator. If it runs fine, I’d put selling it on the back burner and sell only if someone really wants it.
My Onan has been good with two exceptions. I had an electrical gremlin in the starting circuit one time in 15 years. It came out of nowhere and went away out of nowhere. I regularly start it and run full synthetic in it.
I got bad propane once with lots of additive/water. It ran awful on that. Once I drained off the additive, it worked fine again. I don’t think that’s an Onan issue. It was a propane supplier issue.