In my 34E Hurricane, it's not actually very loud inside the coach. The furnace is located underneath the refrigerator, and behind cabinets. The 1/2 bathroom's location also helps to insulate the sound from the rest of the coach.
When sleeping, I sometimes crack the bedroom window. A couple of times I have been awoken at 3 a.m. by the external sound of a tiny F16 afterburner kicking in. The externals furnace vent is about five feet from the window. With the window closed, even the tiny jet engine isn't very noticeable.
From years ago with my Fleetwood Folding Trailer, (a PopUp camping trailer), with it's haphazardly mounted furnace I learned that mounting is everything. Those mini-jet engine furnaces must be securely mounted to solid frame, (and/or walls and cabinets). If not, they will resonate and greatly amplify the sounds that they create. After I very securely re-mounted the PopUp's furnace, it became less noisy and more hospitable.