I had the same problem after filling up.
After much searching ,it turned out to be the filler neck vent tube.
The vent tube is sweat soldered in side of the neck a couple of inches down from the cap.
It had cracked where it was soldered to the larger filler tube.
It looked like someone in the past had damaged it when installing possible a replacement hose?
I removed the complete upper assembly from the hoses and re soldered it.
Hope yours is that easy
It's hard to believe such a small crack can cause such a BIG odor