Sounds to me (from reading on here and other forums) that when an Onan poops out, usually it's from poor maintenance. Most all the comments I read are about failure to start but no one ever comments about how it was mainbtained.
I have an Onan opposed twin engine in a small bobcat I own and it's extremely reliable.
If you don't take care of something mechanical and take it for granted, it will always let you down when you need it most.
Far as fuel consumption goes, cannot comment other than to say the Onana I have is pretty frugal on gasoline.