Other Fuel System Tank Assembly problems of the 2012 Chevrolet Silverado 1500

I have a 2012 silverado that had an engine light go on. Took to the dealer (carbone chevy, yorkville NY) the problem was an air leak in the fuel system. The fix, replace gas tank and sending unit, $900. I asked about the warranty, and I was told past mileage, I'm at 39000. I didn't believe this could be possible that a 4-year-old truck could rot out so soon that there must be a problem with these tanks. I called gm to ask, and the woman could not have cared less just listen to me complain and said she would call the customer experience person at the dealership to contact me. That didn't happen. Also said that the corrosion warranty doesn't cover tanks and it's up to me to pay for this. I spent $36000 on this truck and didn't have a scratch on it but now find out it is rotting through like a tin can. My questions are; should this have been covered or recalled? is there anything you can do to help?.