I just traded a 2007 Class C Forester 31 foot with the Ford v-10. You can expect 7-9 miles per gallon. I towed a 14 foot trailer with a Harley Trike (7mpg)
With no trailer I got 9mpg. The best mileage is achieved at around 60mph or slightly under. This mileage was on a mix of level and mountainous highways.
The Ford v-10 is a good engine and has plenty of power.