Another thing I often notice about Onan generators. A lot of RV makers mount them very poorly. Ideally, there should be mounted with rubber shock/vibration mounts, the exhaust also should be mounted to minimize vibrations, as well as the fuel hoses and wires. The area between the top of the generator and the van should have some type of insulation, be it Dynamat insulation that is resistant to heat and such, or another acoustic format.
A good installation can take 10+ decibels off the noise of a generator with ease. I've seen this done with some of the larger Onans which were running quieter than my 3000 watt inverter Yamaha.