I don't know where you got the 7100 tow rating for the V-6 engine. According to GM's trailering guide the maximum tow is 4400 lbs. with the extended or double cab. It goes up to 6700 lbs. with the 4.8L V-8, and to 9600 with the 5.3L V-8 and the 3.42 gears (only 6900 with the 3.08 gears).
Fuel consumption for a tow vehicle should be the last thing to think about. If you take a trip that covers 1000 miles and one engine provides 25 MPG and the other provides 18 MPG the difference in gallons of gasoline will be 15.5 gallons or about $60. If you drive 15,000 miles each year the additional monthly cost for gas will be Over 50,000 miles the difference is still only an additional $75 each month.
The place to save money is at the car dealership when buying the new truck. Trucks are more profitable for the manufactures and the dealers than even the luxury SUV's and people pay way too much and a lot more than they need to for their trucks. If you buy a vehicle at the dealer's invoiced cost they still make a good profit on it. MSRP and factory incentives and zero interest are targeted at people so they can be suckered into paying a lot more than they need to while still leaving thinking they got a great deal.
