I tried to fit the Viair 2 gallon everywhere on a 12 but it ain't easy. There is a hose in the way for the behind the radiator so the bracket for the 3.8 won't work. The cubby in the trunk is too short. Even with the plastic removed you would need to build a cover that gives a couple inches of clearance. The Viair site has good dimensional drawings for all their tanks and all the other tanks I found were at least as tall, about 9" if I remember right. The one possible location I found would be above the rear axle on a lifted rig but I was too lazy to pull the shocks and springs to measure it.
I ended up bolting it to my Tuffy security deck.
I made a plate to attach it vertically next to my spare and will bolt it to my tire carrier swing arm when I get the time. It should fit nicely. The 2.5 gallon is longer and the 5 gal would stick out more than the tire so really the 2 is the best fit there.