I have the 3.73 gear and have no problem with my trailer, only 9000#.

If you want better MPG, you will have to be easier on the gas pedal. You can get around 14-16 on the highway if you keep it around 65-68 with the cruise. You should not expect good gas mileage with a 7500 lbs truck that has a flat front end.

As far as the tuners, I have one and it made a big difference in the way the truck reacts to the throttle. Have not seen much increase in the fuel economy yet, but I have only used the tow and performance tunes so far. Won't be using the economy tune until Sep. I do seem to get a little better with the tow tune then the performance, mainly because the performance tune is fun and fast. I would not recommend you getting a tune just for MPG's, you will be disappointed.