towing fuel milage

I'll state it another way. Every gas engine vehicle will use very close to the same amount of fuel when the aerodynamic drag is the same, i.e. same high walled trailer. So like the OP, I would expect nearly the same mpg from the current V6 as the prior 4-cly assuming the same driving conditions. So my theory is that the driving conditions are not the same, i.e. he is driving faster since he now has significantly more power/gearing/wheel torque available. If he slows down to the same 35-40mph up those hills, then I expect the fuel to fall back in line. If he is holding 68mph up every hill and freeway overpass, then it will cost more fuel.