I would have the exhaust system of the generator checked. Your's could have a similar problem to mine. Gaskets are cheap, pipes are not, but what is our life worth?
I have the same problem with my recently purchased coach. I think it could be a bad gasket on the exhaust pipes or even a fractured pipe. I won't know until I pull the Onan out for service.
One of the previous owners made the Onan a permanent resident and I have to cut out what they built and then fabricate a new generator slide for mine.
1988 Coachmen 32 Ft. Sportscoach Pathfinder DL S
Class A gas, Chevy P37 Chassis 7.4L Engine
Retired Army, Swansea, South Carolina