It will be driving style and road/load/wind/grade dependent, but on the highway you should be able to get at least 8 MPG or better if not towing and keeping under 65 MPH. Which engine is in your coach? My 2007 Tour Master with the Mercedes turbo diesel got just a bit better mileage than the Cummins version...
2007 Tour Master T40C
Acura MDX toad
"It takes a great deal of time to recover from any improvement..."