You may have a filler neck issue. If the issue is more pronounced when you have a full tank and the check engine light is on - that's likely what's going on.
The fuel filler neck is a part that is often overlooked until it's gone bad, or a check engine light prevents the car from passing an emissions inspection. (this is generally true for cars made from 1996 onwards). I've seen a fair amount of GM fuel filler necks develop leaks from rust on cars in northern climates over the age of 7. One way to determine is to fill the tank of your car with gas, leave the gas station, and park the car. Then walk around the car sniffing around for the smell. If the smell is worst near the filler door - the neck has a problem. Since the gas tank is right below the rear seat and there are interior drains in the vicinity, it isn't unlikely that the smell could work its way in through there. Or, there could be more corrosion.