2002-2007 Dodge 2500 or 3500. If you get an automatic transmission - make sure power (intake, exhaust, tuner, etc.) was not upgraded. 6-speed is okay if slight power was upgraded. I would go with 3.73 gears instead of my 4.10. My lie-O-meter says 24mpg but it's really around 19-20ish on an 8,000lb truck. If I had 3.73, I bet I could get much closer to 24/25 if not better.
Put a Topper on it, and then keep it stock height, and a set of decent tires.
I have a 2006 Dodge 2500 Megacab 6-speed with a topper on it. The 6.5' bed is plenty of room for dogs and luggage tossed into the back. The megacab is HUGE for interior space, the rear seats lean back and I still have room for stuff behind them. I do an Oil change once a year since I don't drive much. A fuel filter change ever 10-15,000 miles just to protect my injectors (fail point), and run a winterizer to make sure fuel wont gel up in Winter. I forgot to plug it in one night and she fired right up the next morning when it was around 0* plus or minus a few. The 5.9 is a legendary motor, just need to make sure the injectors are cleaned. Long term reliability, plenty of parts around the nation, and is just a brute. I'll do diesel for the rest of my life after owning and learning about them in school.