Nice lookin' set up!
I'm ave around 1200-1300 miles per gallon of DEF. I guess realistically that would equate to 2 fill ups per year for me (capacity=5 gallons).
Is your truck a short or long bed? Ours is a short bed and I agree that the stock tank IMO is worthless when it comes time to tow. If you figure 10 mpg on ave, you have to stop every 200 miles and this leaves you w/ just a few gallons in tank before you call road service. Replaced stock tank w/ a Transferflow 50 gallon and feel much better about having extra fuel. No one has complained (yet), but I like to go at least 300-400 miles before needing fuel.