I have a 2003 duramax 4 door dually and I tow a trailer long distances on occasion. I need to pull the best fuel mileage out of this thing I can, hence the reason I went to a diesel. So far just has a volant cold air intake. Going to a straight 4" exhaust. What else can I do reasonably? Is there a decent tuner (or tune) I can run on the truck to help? Any other ideas?