I try to call it like I see it. There are some members that I have agreed with in the past that I sort of disagree with here. So please, nobody take offense...
Yes I understand that the Engineers across the board are placing smaller tanks in these trucks for EPA and CAFE standards and I fully comprehend that move. Range is roughly the same as it has always been because the newer trucks are more fuel efficient. But I don't support knocking this guy because he wants a bigger tank. This IS a forum, after all. Someone in another thread knocked one of us for having a high lift. But the man can lift as high as he wants. This man is just reaching "up" in his own way, I guess.
My GMT400 1993 and '94 full sized, 2 door Blazer Silverados both had 30 gallon tanks. And they were much shorter and more maneuverable than a current Sierra double cab with 6.5 ft bed. But they got worse mileage. The Suburbans of that same period had 40 gallon tanks. In the square body generation prior to the GMT400, an option was available to have an entire second tank installed at the truck's passenger side! When your fuel gauge read low, just flip a switch and watch it climb back up. If they both read low, then the wallet had to really strain.
The man feels like he's being gypped even though he isnt. He wants more, and I guess that's not wrong.
I wish my 14 Sierra had a 30 gallon tank, or the option for a second 25 (24?) gallon. But it doesnt. I can still get 550, maybe even close to 600 miles if I really hyper mile it on flat ground. I wish I had more but I'm coping.
Maybe the OP could redevelop the 2nd fuel tank system and sell it, and see how many takers he gets. I'd like to see that.