I would check the top of the fuel tank. I had a truck that when you walked by it you could smell the gasoline. Turned out to be the top of the tank had a rust hole about 1/2" diameter. Since it was on top, no leakage, and never got water into it, since it was covered by the body. Found it by filling it to the top, and into the filler pipe, and then it started leaking when I drove it up a steep hill. Drove the truck till the tank was just about empty, and I had a replacement to bolt it the same day that I removed the bad one.