If it was me, I would probably buy NEW. If the old one is repaired something else could fail like engine parts but not probable with the low hours.
You have used it less than an hour and a half per month so I might look for a cheaper alternative to Onan.
Marvin
2010 Coachman Freelander 22TB on a
2008 Sprinter/Freightliner chassis
1995 Geo Tracker (Toad)