That bare metal look is a weight savings.
I remember reading about one guy who never, ever washed his truck, he just dumped the basin over the truck every time he changed the oil. :O Forty years later, no rust. No paint either, but no rust.
21 mpg is fantastic in a '66 truck, though I think in this case yours is more a 2011 with a much nicer body.
Start a garage page for the beast, and we can see how it does on an ongoing basis.