First thing comes to mind is whether it leaks all the time, or only when the fuel is above a certain level. That might help identify what part of the tank you need to concentrate on.
The problem with finding the leak with air pressure is that the line is not an overflow, but the tank vent, and it connects to the "burrito" in the upper part of the tank. The "burrito" has several openings to allow air and some fuel to enter and leave it. Obviously, these holes will make it difficult to spot an air leak in the tube.
As far as the probable location of the leak, the first place to look is the spot where the tube exits the tank. Corrosion or a broken weld could cause your problem. Fortunately, that would be the easiest to repair. I really doubt that there is a hole in the tube as I can't imagine a way to make one where the tube runs in the tank.
It is also remotely possible that the problem is a bad o-ring in the plate that holds the fuel level sensor. There's a slight chance that the tank insulation is guiding the leaking fuel from the plate to the spot where the vent spigot is located.