It could be that the tank vent is being blocked somehow and fumes are getting back into the car. Check the vent which should be near the fuel filler neck and make sure it is not blocked or broken open. Then check the one way flaps that also vent fumes which should be somewhere in the rear low and near the filler neck and there might be one on each side. You don't have to see gas for there to be a leak. You at leaast know it is not a pressurized leak as you would see gas then, but not always as it does evaporate quickly and just a little gas can smell a lot. Also, you might try getting a new gas cap as it may not be venting properly.