Congratulations on your new ride. We had a '15 GMC Duramax dually for 3 and 1/2 years before going to the dark side. The only issues we had were a minor oil leak and an emissions issue with the DEF tank, fixed quickly under warranty.
I changed out the stock fuel tank for a 60 gallon Titan replacement tank but I'm not sure Titan has them for the new GM models. We could tow over 500 miles without stopping for fuel and did not lose any bed space. Our new Ford has a 48 gallon tank and I wish it was bigger. I also changed out the stock shock absorbers to some Bilstein 4600's which made a noticeable difference with the empty and loaded ride.
Have fun!