I do not answer to expert......but I have a few hair brained ideas..
My plan is to line the inside of the generator enclosure with a product such as KilMat or MAT66. These are self stick sound deadeners. Also, this can go inside any cabinets or closets nearby that may bring sound into the living area.
Also plan to add a Gen-Turi which is long exhaust pipe to vent exhaust up above the roof. This will take some sound with it as well as exhaust.
Also, my understanding is that the sound or the perception of sound can be lessened by making sure the rubber mounts that hold gen to the motorhome are still rubbery and not becoming brittle. The generators attachment point can transmit deep bass tones that are not heard if you can isolate the generator.
To that end I may try to mount the generator on a rubber pad and the pad will be mounted to the motorhome.
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