It's possible, but I couldn't imagine a Loss of 10mpg resulting
Just from the a/c
Wow a light just turned on upstairs (it's a light I thought was broken, so I better get this out quick) if it runs well, and there's lots of power (no loss of power) logically it doesn't seem like a problem with the truck it's self.... Do you maybe have a fuel leak somewhere?
Check the fuel reg, pull the vac line off and put a tube on it and suck, (engine running) see if you get a gas taste in that tube....) that could cause a drop in fuel Econ, with minimal/no performance effects, and no smell...
Fwiw, my ranger's fuel neck was leaking and I couldn't smell it, same with my 250's rear tank, leaked like a siv, I lost a half a tank of gas over night (though I saw the massive puddle and could hear it draining gas...)