Where to fill air in dually rear wheels?

I do it in my driveway, using my big shop compressor. If you're needing to add air more often than once every several months you have a leak that should be attended to. I find I drop a few PSI over about six months, so twice a year suffices, primarily once in the late fall when the cold weather arrives. I don't worry about the difference between 75 and 80 psi, I doubt that my air gauge is anywhere close to that accurate.

I've never see one one of those cigarette lighter compressors that wasn't useless. You could grow old waiting for it to pump up a truck tire.