I've had two starting/running problems with my Onan, vintage 2011. First was the oil level switch. With a full oil sump the gen. was running rough and was hard to start, then failed to run. Long story cut short, disconnected the oil level switch, gen. starts very quick and runs as smooth as new. Second problem, gen. would crank but not start. With the front access panel removed, the carb is in front of you, there was a small spring on top of the carb that was disconnected, hooked up spring and she fired right up.
Possible oil leak could be the oil level switch. It screws into the sump. Location is to the right of the oil fill tube. On mine the wire to the switch is tie-wrapped to the oil fill tube. When the switch senses loss of oil level its contacts close grounding the circuit.
Maybe this will help.