Ram tow vehicle
A 2004.5 Ram diesel will have the better 48RE trans and more HP while the early 2004.5 may have less HP and I believe a 47RE trans. The better stuff was optional at first.
I have a 2004.5 4 door dually, auto and 3.73 gear with about 105,000 miles. It has been flawless towing a 26' or 28' Pace with an overall combined weight of just under 20,000. Small chip, I think 125hp. But I also do maintanence religiously. Best mileage is around 1,700 rpm/60-65 mph. Amsoil synthetics throughout.
2003 gets better mileage because it doesn't have an extra injection cycle. Not sure how mileage is with the late 5.9's or 6.7's.