My non turbo 6.9L IDI motorhome (11,000 pounds) with a 3 speed transmission gets 11-12mpg cruise control @ 60mph on a nice flat highway. It does have highway gears tho, 3.54

That PS van might have a really low geared rear end to have that kind of mpg, or somethings wrong. Maybe just in need of a tune up.

What's the mileage on it?