So much depends on how you drive, how often you stop, how many grades you pull, how well the tank was filled from the last run, etc etc.
As we are Geocachers, we tend to stop a lot, and work our way through rural routes which takes a toll on mileage. Yet there are times when we run for many miles on the interstates, yet the other factors are still in place.
You'll see numbers all over the place. We consider 9 to be average for our 27' MH with no toad, 2 to 3 passengers. We've seen tanks in the 6's and tanks in the 12's. We have 42K on the clock.
Here's a chart from our trip this summer: