The truck in tow/haul mode  will not shift past 4th gear.  It makes no difference what
speed you run.  In manual it will work properly (great) for a 100 miles or so,It has
extremely good power and operates with good gas milage.  Then it will not shift into
6th gear unless you lift the accelertor,  when you push the acelerator again it will shift
back to 5th gear.  It has limited power and uses much more gas.

I have taken it to the dealer twice and they say it is fine because no light comes on to
indicate trouble. I have pulled a trailer 4,000 miles and burned 27 tanks of gas.  
Three times it got 10 miles to gallon, then it was 6-8 mpg.  Same driver the entire
time over variety of highways.