To add to all of this- the giant rubber grommet that seals the gap between the filler tube and the cab doesn’t always provide a gas tight seal... if the gas dribbles over the filler neck, even a little, it can sometimes find its way into the cab. Find a good product to seal the grommet all the way around the filler tube. You can seal it inside the cab, where it won’t be noticeable. Solved the problem on my ‘63 GMC.
