When it comes to noise from an onboard genny, the main source to the neighbours, is the exhaust. How do you reduce that?
My 5er is so-called genny prepped whuich means the cables are there & a tin genny enclosure is in the basement. NO sound insulation installed whatsoever. I hate to think how much noise the genny would make, internally & externally, if one was ever installed.
I have experience with a boat genny. Have a 6KW diesel genny in my boat. A typical marine genny is heavily sound insulated. It does not have a radiator which needs to draw in & expell air & is a source of engine noise escaping. It draws cooling water from outside through a heat exchanger then ejects it into the exhaust system, which helps quiet the exhaust. The only open exposure a marine genny needs is a source of air from outside to run it.
Unfortunately RV gennys cannot be sound insulated that way. They can be insulated to reduce the noise to the occupants inside but those around them suffer.
The best solution is an inverter type genny which most of the time won't be running much over an idle. Trouble is they are not cheap.