Boy lots of info for you. Honestly I would go to a Duramax forum and ask the question over there, you're gonna get really good information from people that have done some serious mods to their trucks. Now as for the more fuel=more heat= less MPG, that is false. You make more power just by changing the timing, cool your EGT's, and get better mileage as a by product. I have increased the RWHP on my truck by over 150HP, have lowered EGT's when towing, and gained 1mpg across the board. A lot no, but figuring all I have done it is impressive. What I have noticed is a lot less hunting with the tranny with the power. The tuners that are added alter all sorts of parameters to make your engine more efficient. If you add a small tune to your truck, you will find a more capable tow rig for sure. Be aware though, you have to "pay to play" for every mod that you do, whether a chip (yes they have chips for D-Maxes, quadzilla or Edge to name a few) or program it you will need supporting mods such as turbos, tranny mods, head studs, etc. For a small tune you should be fine, but I wouldn't tow on it without gauges and beefing up your tranny.
In answer to your original question, EFI live is the best tuner for your truck, and tranny BTW. Next I would say is PPE (Pacific Performance Engineering) They both specialize in D-Maxes. Good Luck