Here's my perspective (FWIW). How much someone drives in a given period of time will cause fuel expenses between different coaches, with different MPG ratings, to be more noticeable. The average RV only rolls about 5000 miles per year so it would obviously be up to each individual to determine what's acceptable to them.
You can significantly affect your MPG in a motorhome by driving slower, but each coach has its own "sweet spot". My 32 foot Allegro with a Ford V-10 consistently gets 9 MPG at 63 MPH. I once drove a long leg through Oregon at 70 MPH and averaged 7.5 MPG. (I'm still working so my travel time on vacations isn't as leisurely as I'd like.)
After owning two Class A's over several years, (and about to buy a third), it's my opinion that a coach's amenities and floorplan are far more important than a difference in a few MPG, but that's a reflection of my priorities, not anyone else's.
One more thing... Like someone above posted, I've never been a Ford guy, but the Triton V-10 in my coach has never caused me ANY problems. It's the lower HP version of what they put in newer coaches but I've never felt that it was under-powered. Our next coach will be a DP but if I were going to buy another gas coach, I wouldn't hesitate for a second to buy one with a Ford V-10.