My van does the same (but diesel). If I fill up then park on my angled driveway, I get diesel dripping out of the lower end.
I have the transfer flow 46 gal tank. There are two roll over vent valves that apparently can leak. I can reach them with the tank in place, but haven't had the nerve to try to twist them out blind to see if I can fit a new o-ring. The guys at transfer flow said to drop the tank to do it. The roll over vent valves are from the stock tank and re-used on the transfer flow tank.
Below is the email I received from Transfer Flow when I emailed them about this.
I attached a copy of the installation instructions. There are two roll over vent valves on top of the tank. The valves came from the OEM tank. The rollover valves have a dust cap on the end. To install the roll over valves they are inserted and turned 90 degrees clockwise one quarter turn to lock them in. The vent or cap will point towards the driver frame rail. If it isnít rotated completely that could cause a leak. The roll over valve has a o-ring that seats into the cam ring on the tank. The valve itself may be leaking or it may be leaking around the o-ring. One way to cure this leak is to (Drop the tank) so you can get a hammer and a punch, bend down the notches downward on the cam ring. This will secure the roll over valve much tighter . in the cam ring. Look at the drawing below.
To seat the rollover valves tighter to the cam ring if that is whatís causing the leak. Bend the tabs downward a little.
The best way to confirm what is leaking is to pressurize the tank a little and spray a soapy water solution to the valves.
Transfer Flow Inc.