I have a 2010 RAM 1500, HEMI, crew cab, 3.92 rear end, with the extra capacity fuel tank. Bought this specifically for towing. I pull about #9000 when rigged and full of water and fuel. (Discovery 246 w/dual axle Venture 8025 trailer.)
Highway towing = 8 mpg. (plenty of power for hills and passing though)
Highway non-towing = 21 mpg.
I've been very happy with this rig.