Bit further out than that area, but Don's Trailer Service in K/W does work on Onans. I had mine diagnosed and repaired there a long while back, when I had a Roadtrek 190P. They're on Shirley Ave in a strip mall, just down the street from the old Roadtrek factory. They were very good back then. You could try sending them a note or give them a call.
http://donstrailer.com/I have never had the generator in any of my motorhomes set off the CO detector. It's probably worth pursuing, and having it checked. If the leakage is inside the genset, like an internal exhaust/manifold leak, it will need further diagnosis. Better to get it checked?