It's not rocket science to stop the coach noise in a Class C (or A, or B). Just go back in the coach areas, look long and careful at everything back there, and then insulate loose items that "might" bang against each other ... whether or not you actually have "proof" that they are.
We use the same approach in our family sedan ... if something on the floor in the back seat or the trunk is rolling around making some irritating sound ... track it down and take action.
It's outside wind noise on the structure that's very difficult to locate and stop ... especially wind noise from the portruding awning roll mechanism.