I was getting 16 city, 13 towing anything in my 6.0. It was up to 24 mpg highway not towing before I lifted it, which was in the first 3000 miles. I should have left it stock height.
BF Goodrich All Terrain T/A's are the Arizona gold standard. Mud Terrains if you don't mind a little more noise.
I'm ambivalent about programmers. I actually saw an improvement with the Superchips one but that's also because Ford kept reflashing the trucks to detune them because of all the rednecks blowing head gaskets. It killed the mileage so I bought the tuner to raise it back up and never had any issues even using the "Performance" setting. I say check on the price of a new crate motor before you go hog wild on mods. Mine was $16k...