I think it is one part build and one part location. If you watch the "Gone with the Wynns", they say that their Bounder needed some additional insulation. Maybe other coaches already do that?
Second, and I think more important, is where you drive. My class C is perfectly quiet when we are cruising on flatlands (much of America), but it absolutely roars when we are climbing mountains and large hills (essentially anywhere west of Denver).
When someone suggests that a particular motor/chassis is (or is not) strong, loud, quite, fuel efficient, etc.... consider where they typically drive.
