Modern diesel pickups carry enough diesel exhaust fluid to operate the emissions control equipment for several thousand miles. The DEF tank on these trucks holds 8 to 10 gallons. Chevrolet hangs the tank outside the frame rail where it's vulnerable to trail obstacles and debris thrown up by the front wheels. Ford places the DEF tank in a similar location if you opt for the larger fuel tank. (I'm not sure where Ram sticks their DEF tank.)
The DEF tank on the current, new Ram pickups (not cab & chassis) is only 5.5 gallons, but it's
enough depending on fuel usage, I went 7k before needing my first fill. I'd like a slightly bigger DEF tank, but I understand the packaging limitations and challenges.
RAM did a good job putting the DEF tank just forward of the fuel tank on the driver's side, with a nice flat metal plate below it, and has a gauge instead of an idiot/warning light (think Ford now has a DEF gauge too, but at one point did not).
I've never been a modern GM truck fan, too low for me, the body, the frame, and now the DEF tank...that thing is an abomination.