I found the local Cummins shop also has all my Onan parts. My Marquis 5500 had annual oil, fuel and air filter changes and still ran on the same plug its entire life (I did clean it annually and checked gap). My little LPG model does not have an oil or fuel filter and burns cleaner than my gasser did, but I still do the same. At some point, I would expect these engines need a valve adjustment but never got to that point.
Major repair would require dropping the generator out of the compartment, but regular maintenance is easy.