On our 26ft motorhome the generator is under the bed. We don't run it at night when sleeping but if stopped during the day for a nap the generator doesn't seem overly loud....no noticeable vibration and the air conditioner masks the sound of the generator.
So (in my opinion) if you can sleep with the air conditioner running in a class c motorhome the generator under the bed should not be a problem. Personally, I would not pass up a motorhome which has a floor plan that you really like just because the generator is under the bed.