What makes the inverter generators seem, on average, quieter is that most campers don't use them at full RPM because most campers don't need their full power most of the time.
Of course, the way less expensive open frame generators are less expensive because they aren't using the variable speed inverter technology. They just run at full speed whether or not the RV is pulling or needing full power from them.
There used to be available small wattage four-cycle non-inverter generators that were not variable speed - they operated at only a fixed high RPM - but were also very quiet because their engines were so small. I have one, and it's just one decibel louder at it's full speed than an inverter Honda is when idling. However, the generator I have is capable of putting out more wattage at it's full speed (and very low noise) than the Honda is when idling.
So ... the inverter generators are not very quiet when delivering high power ... they're just quiet when being used as a very small generator.