Use the tow haul mode, it's there to protect the transmission as much as to improve towing. Less shifting, higher shift pressure, different converter lock up criteria, etc.
As for improving MPG, a lot of what you hear is bunk. I mean, all those advertisements that claim "Up to 30% improvement in MPG and power!" really must be drinking good stuff, otherwise I'd install three of them and nearly double my power and MPG!
Keep your injectors cleaned every 25-40,000 miles, keep your oil changed, keep tire pressure at spec, when it's tire replacement time pay attention to mileage claims as tires can make a HUGE difference in MPG especially in truck tires. BFG Mud T/A's look awesome aren't the hot ticket for decent MPG but the All Terrains are better and Long Trail TA's are better still.
Dropping a few MPH helps more than I can say. I lose two MPG towing at 70 vs. 60.
And, don't just leave the cruise on. On the flats, sure. But climbing a hill? The truck will floor it to maintain speed where you would back off a little knowing that 5-10 MPH isn't that important.
It's all about being efficient.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10