I have a '93 Fleetwood Bounder which I purchased new and it also had a noticable whine right from the start. This occured between 57 to 62 mph. I am pretty sure this is a differential whine and not anything to do with your coach body. There is probably nothing you can do, just the luck of the draw. In my case I just avoided that speed range, which was not a great inconvenience because I preferred 55mph as my top speed.
During my career as a machinist I cut many gears of all types and in an assembly it takes very little variance to produce a whine sound or resonance at a certain rotation speed and load. The noise can be very difficult to diagnose. Most likely it is the mating of the ring and pinion. In theory it will be their forever, but over the years and 200k miles mine actually quieted down very slowly and steadily. which does not make sense to me. With wear one would think there would be more chance of vibration. If it is your differential gearing it will probably not affect longevity, like abuse does, just a big annoyance.
I would take it to a Ford truck dealer and ask them to test drive it and get their reaction. Maybe you can get them to swap out the gears. No amount of adjusting will fix the problem in my experience. The gearing I am familiar with was not associated with automotive applications.
Does the whine occur under light load and change under no load or heavy load? If it does I would again think pinion gear. There are technical papers you can find on the web which explains our problem, I will try and find one and post it.
I would avoid automotive posts on forums about this issue, too much misinformation might be found, like mine above perhaps.