Ah, I read too fast. The one picture in the eHow site shows a return air register in the floor, which means that particular RV has duct work. But the site is simply talking about adding sound dampening panels, as described in the RV.net thread you referred to. Adding such panels will reduce the noise coming through the couch/seat in the TM, but it doesn't do anything to reduce the noise coming through the fan grate -- and I think that's where most of the noise is coming from.
Backing the furnace away from the front of the couch/seat and creating a squirrel cage like pictured in the RV.net would be much more effective at reducing noise. But that also is going to reduce air flow and heat, and requires a major modification because the outside vents would also have to be moved.
Obviously, the major source of noise is the blower -- I think there are likely much quieter blowers out there. I don't think I would try to retrofit an existing furnace, but my guess is there are quieter (and more expensive) RV furnaces out there. You can barely hear the most powerful bathroom exhaust fans at home depot, and the cheapest ones sound like a hair dryer.
Have you searched for quiet furnaces at all? This one claims to be "whisper quiet", although I don't think I would buy one unless I saw one in person, or at least a video:
http://www.rvpartscenter.com/Product...DID=84&CID=288
Dave