I have had a motorized RV for 20+ years, last year I bought a Roadtrek RV with a good running Onan generator. I removed it and sold it before it could cost me money! Been using Honda for 20 years!
The main problem with any carbureted engine is the ethanol in the gas, the gasoline evaporates and the ethanol gums up the carburetor. This applies to lawn mowers, weed wackers, chain saws etc. Also the various electrical components, again any generator not just Ohno can corrode. The Onans can be a money pit if not cared for! There is a lady on the generator forum that has spent $1000 on an Onan that was not used for 3 years and it still is not right.
If I were forced to live with a Onan I would run it at least 30 minutes twice a month with a load, AC, electric heater, something that loads the generator not just a light bulb or radio. If you want to store the RV for months, there are instructions online on how to prep the generator.
My opinion and worth what you are paying for it.