Short of replacing the gear set there is not much to be done, take an oil sample and have it analized , that can help determine if it's the diff. making the noise.
But before you go that route; while driving, take carefull note of the RPMs of the motor and the gear the trans is in. If your in fifth gear ; for example; when the noise is most evident, then slow down, shift the trans into forth, and operate the motor through the same RPM range. If the noise appears to be the same. Then it is quite likely turbo related.
A few questions, is this a new noise , or are you new to the coach?
What engine , trans combination are you running ? Any recent changes to the coach; IE. muffler? air filter ? Tires? . The reason I ask , is that many people who change to a K&N style air filter hear a whole bunch more engine noise than they did with a stock air filter.