If it makes any difference, the class-A we rented a couple of years ago had an amazingly loud furnace too, and it used a ducted furnace.
I think you would need a much lower fan speed (or at least lower air speed) to make it any quieter, and I suspect that the massive airflow across the heat exchanger is needed to keep the heat exchanger from getting too hot (pinholes in a heat exchanger == carbon monoxide in the RV).
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