The ad says it has a Detroit Diesel, and a 5 speed manual transmission. I assume that's a 6-71. It should get better fuel milage than 5 mpg. It's a mid engine bus, the engine laying on its side. If it was a turbo'd 6-71, I'd consider it for a few thousand. You better be a gearhead though, & those Crowns have those funny screw heads on the chassis fasteners.
Buy the bus, your life will never be the same.