Speaking from a different angle here - I'm rather spoiled by my Honda EU3000is that I camped w/ when I had my travel trailer. Talk about QUIET and fuel efficient.
The Onan 4000 on my Winnie is pretty much just slapped inside a metal box aft of the driver's seat. Its loud. I'd almost rather they didn't put a generator in there at all and save me the OCCC as I never camp w/o full hookups. I couldn't imagine sleeping w/ all that Onan noise. About the only comfort I get from my generator is being able to run it while underway to keep the fridge going. And MAYBE knowing I'd have power if the campsite lost theirs.
I'm sure other rig's have a bit more sound proofing and maybe even a bunch of rubber dampers and all. However, you can't touch that EU3000is's 58db at full load.
Should've never bought that thing. Now I'm spoiled.