Ditto for getting a RDs tank from N Tool, I have a 51 gallon alum tank. It also includes tool box, that I love. Installed it myself with the gravity-feed attachment that NT also sells.
My 2009 D/A 2500 only has approx a 28 gall tank. Having the aux tank gives me right at 80 gallons total (at roughly 10mpg pulling 34" 5w, can drive close to 800 miles).
I never fill aux tank up, only put in what I think I will need for the day's drive (mostly drive between 325 to 350 miles). I usually add about 15 gall of diesel in the aux. tank. This gives me a fuel cushion & helps keep the weight down for my R axle.