new diesel owner

I just bought a 2004.5 duramax 2500HD. It has a 6" lift with 35's. The truck is everything I wanted and more. It just rolled over 100k on the way home from purchasing it. I am not an over heater as of yet and want to avoid being one. I bought the truck to tow a 22' camp trailer and for work to haul my job box from job site to job site.
So I guess my questions are; What do I need to do to keep from over heating, and what do I look for to see if its alredy been done?
The truck is origionally from California and has holes where the 5th wheel hitch use to be in the bed.
Also I would like to get a programmer for the truck, the only reason I want to do that is for the towing, and fuel milage. I dont need a bunch of power that I dont plan on using.
Any info would be helpful, thanx!