I had the same problem with my new gas tank. I can't remember if I got it from Chey Duty or not. But it turned out the gas was leaking around the large washer that is spot-welded to the inside of the tank, not the threads on the nipple that screws into the washer. I took it to a radiator shop and had some solder put around the washer ( since it had gas fumes in it, he used a giant soldering iron instead of a torch). That stopped the leak. I know I should have sent it back, but I was too impatient.

My dad always told me, "Don't force it, get a bigger hammer".