24 Valves use more fuel than 12 valves do, and in 98.5 Dodge made the (bad) swap to the 24 valves. So your 99 has the 24 Valve, and watch out because the 24 V's go through lift pumps fairly often. You didn't specify 4x4 or 4x2, manual or auto, but with 13-15mpg I'm gonna guess its 4x4 auto. Automatics also get a little less mileage than manuals do.
One thing you need on a 24 Valve <font color=blue>Cummins</font color=blue>(not Cumming) is a fuel pressure guage, along with Boost and Exhaust gas temperature guages if you tow a lot. Also if you truck is an auto and you tow regularly invest in a good tranny cooler. It will pay for itself because Cummins love to eat up Automatics.
I don't tow very much with my 97 but I still added the Boost and EGT guages to key and eye on the motor.
A good site to check out is <a target="_blank" href=http://www.turbodieselregistry.com>www.turbodieselregistr y.com</a>. It cost's $35 a year to join, but its well worth it. You can go through and read the discussion forums without being a member, but you won't be able to post. Look for my username "Got Smoke?"