Gas fumes in car

I am writing for a friend who has a Jeep Cherokee 1990, almost 200,000 miles with a 6-cylinder engine.

She is experiencing strong gas fumes in the car, but not all the time. She realizes now that it seems to be only when getting gas at certain gas stations (Shell, for one) and the fumes remain until the tank is empty and she puts in a different brand. The gas mileage is also a lot worse at the time she smells the fumes.

Is there any reason why she may experience thiis at certain stations but not others? If it were a gas leak, I would think it would be consistent.

Any information would be appreciated.