Hmmmm .... our Onan 4K has been fine for 10 years in our 2005 motorhome .... but we only have a few hundred hours on it.
However, I do use the most expensive synthetic oil I can find in it and a "special" spark plug (laugh all you want ;) ) in it. It starts right up with minimum cranking, hums along vibration-free through the coach floor, and fairly quiet without sounding labored ... as a result. So far it's well worth the $3K+ adder we had to pay for it included in our rig when we bought it new.
I really appreciate one of it's "features" which I never see mentioned in the forums - a simple to reach and convenient altitude adjustment knob.