Onan is partnered with Cummins.(yes the diesel fame Cummins) In Denver parts are readily available at Cummins Onan Rocky Mountain
http://www.cumminsrockymountain.com/
Anyone who is serious about generators and relies upon them gets an Onan. They are available in Gas, Diesel, Propane, Natural Gas and Kerosene. They are not cheap.They make generators for the Military too like the MEP002A. They are generally not many used available. They aren't just for RV's Most commercial boats use Onan generators too.
They are American Made. They make generators from 2.5kW up to 2MW.
RV generators only go up to 12.5kW
Mobile units range from 2k to 35kW
They have permanent mount residential models too.
You can see the different models and specs here:
http://cumminsonan.com/
BTW I do not work for them, but have serviced them for over 10 years. Comparing an Onan to Generac is like comparing a new Mercedes to a 1971 Alfa Romeo. The Onan is refined, reliable and thought through.
Most service related issues I have experienced with the Onan is operator error or neglect. For example not setting a gas carb for altitude. (BTW it has a dial to set for altitude- no tools needed) Not changing the oil, etc.
I am not a fan of the old Emerald series, but they still work reliably.