You should be able to hold one lower gear on flat highway at 65mph, around 2200rpm in 5th. You have similar gearing to me (if you have 3.73 axle), similar torque, and I have no problem holding 65mpg in 5th. Many people complain with the 6-spd trans prior to Selectshift that the trans hunts for gears a lot. This is a problem especially under load like towing as it generates more heat that holding any gear.
This scenario is typical. Accelerate to desired speed, trans shifts to 5th, then 6th. Truck slows down. Driver wants to accelerate which causes 2-3 gear downshift.
Per your OP, there isn't anything under about $1500 that will increase power noticeably under 3000rpm, even 2200rpm.
Buy a 2015 Expy with Selectshift or 2009+ GM SUV. ;)
