Oh my. You NEED an in-bed extra fuel tank. That's a major benefit of being diesel. Much safer and reasonable to have a tank inside. Would not tow without one. Actually, would not run without one.fuel tank less than 24 gallons? Uhg... Even with our 26 gallons, there were places that I was wishing I had a jug of fuel in the back, like down to a couple of gallons when filling up. You do not always get 30 miles to the gallon, aka, tow mode.
5 1/2 gallon DEF tank. No big deal there.
Engine brake a nice plus and I like the inline 6 cyl motor.
But if it rides and has seats like my last truck, a 2011 Silverado, I would pass. The Ram 1500 is hands down very, very comfortable.
Check out the RDS models.