I have an '06 v6 and tow a boat that is about 4000lbs on a light day, more if loaded up for camping. We also often have a lot of people/gear in the truck.
I find the WORST MPG for me is when in 4th gear at about 60-65mpg. Here in Kentucky we have hills, and every hill will unlock the torque converter in 4th with that much weight (you can see this as a rise in RPM while staying in the same gear, but not going any faster). If I bump it up to about 72-75 I run higher RPMs all the time, but the converter stays locked and I pick up about 2-3mpg. A slipping torque converter makes tons of heat and should be avoided.