We have our Class C setup almost like what the OP wants ... a built-in Onan and we carry along a very quiet Honda 650 watt suitcase generator. The little Honda's noise level is 54 dB at all loads it can handle and runs 5-6 hours on 1/2 gallon of gas. These two gennys together make for a very flexible RV drycamping power mix that doesn't require the sun. The Onan is not noisey enough to irritate and does not vibrate the interior of the coach. We have drycamped with it running the air conditioner almost all night while we slept.
Regarding any fumes from running a built-in Onan ... whenever we run it for any length of time it we close up everything in the RV except one reversible powered roof vent. We set it's fan to AIR IN on the lowest setting so as to slightly pressurize the interior of the coach. This simple trick keeps all Onan fumes out of the interior because they cannot enter due to this air pressure - it doesn't take much. This trick also works to keep all dust out of the interior when traveling on gravel or dirt roads.
