Hey Buddy, I had that same issue for almost 6 months. Then my shop realized, it was the gasket underneath the back passenger seat on the right that needed to be replaced. I got that done & wow, no more smell :)
Check your fuel filler neck. It may have rusted and some fuel splashes through this small hole in the neck. Fumes from the tank will also escape through this gap..Do you have a check engine light on? Also check to make sure all hoses are attached and not cracked in this area.