you only lose 1mpg towing and get 13? i am impressed. I suspect4 you have a pretty high(?) ratio rear end like a 3.75 or 4.11? I agree that ford and GM have a ton of highly skilled engineers who design their trucks and if there was a way to kick gas mileage up any significant amount without losing something else they would do it and shout their a MPG over every TV ad. I just bought the ford 3.5 ecoboost and i average 18 around town and on the interstate not towing. if i do 45-50, i am looking at 25mpg so 2-lane blacktop roads are real attractive. my previous truck was a f150 5.4 v-8 and it did 16. I put on the 3" flowmaster cat back and maybe got 0.5mpg, and then the cold air intake from K&N. that was waste of money. no mpg and all it did was remove the resonating chambers on the stock one giving a REAL powerful howling noise especially with any real throttle opening, like when towing. Deafening! took it off and almost gave it away. Chips? look great till you see you often now need to run high test . Loke THAT saves you money.
best bet, new truck with enough motor for your tow(the extra power doesn't hurt mpg by much when not towing), a sane rear end ratio(i found 3.55 a good compromise for me) and as many transmission gears as you can get(like the ford 10 speed). i do about 2000rpm at 80 and the ecoboost still goes over overpasses with out a downshift.