This could just be a bad seal or line on the top of the gas tank.  Next time
you fill it up reach through the wheel well on the side where it is leaking
and check the hoses that go to the plastic expansion tank there in the wheel
well.  Now feel on top of the tank alittle.  If you feel wetness then that is
where it is leaking.
Ken Wilford
Van-Again
John 3:16