Took a quick look at the Onan RV QG 4000 generator today to see about changing the oil and replacing the oil and air filter. Air filter is readily apparent. But the oil drain is inside the housing and didn't see a filter.
Search of online manuals was inconclusive. This doesn't seem like something that has to be done at the dealer. Any one do this at home?
(edit) Did a you tube search
https://youtu.be/nKPpm-L41Vs