Frame Rust problems of the 2013 GMC Sierra 1500

I went underneath my sierra today and found the entire frame is extremely rusted. Upon closer examination I found a 3 inch by 3/4 inch hole rusted through. This truck just turned 6 years old and has 100000 miles . This is a saftey issue because with the truck being able to tow heavy loads.

On 3/4/2018 I took my truck in for a broken leaf spring. When the mechanic was replacing the springs, he notes holes rusting in the trucks frame. I did buy the truck used from a Chevrolet dealer on 3/12 2016 with @49,000 miles on it. With in 6-8 months of owning the truck, it started to rust on the fender wells, and had some rust on the frame. When I talked to the dealer they said there is nothing they can do for me because the 6 year or 100,000 mile warranty ran out on 10/1/2018. I called GMC customer care about this and they did open a service request # 9-5127228506. On 4/8/2019 service manager for the dealer told me the gme will do nothing for me because it will cost $12,000. 00 to replace frame bet may ofer me a certificate for $1000. 00 towards a new trucks. I fell this is unexceptionable to me because the safety of me and my family when we are in the truck. We brought the truck because in has a good safety rating be with holes rusting in the frame I fell my safety is compromised every time I drive it.