Mileage posts always puzzle me :)
1,000 gals at 4.50 and 7.5mpg is 7500 miles for $4500.
and at 8.5 is 8500 miles, so an extra 1,000 miles.
Not many of us get to drive our rigs that far in a year, so more likely 2yrs.
So other than fulltimers, the cost difference over 2 yrs is a couple of hundred dollars.
Your tires will cost more, your camping spaces will cost more, your insurance will cost more, and your tow setup will cost more.
Mileage isn't the issue for most folks.
Jim