500-1000 pounds of tow capacity or tire/wheel size difference between any OEM sizes will have an insignificant, immeasurable difference in towing power to the extent of wheel/tire combo weight is similar.
Towing any high walled RV is hard work. That's why we get 8-10mpg vs. 18-20mpg unloaded at 65mph. IT takes a lot of power to pull the parachute, err, TT through the air at 65mph. Find a hill and you find out how much reserve power you have.
2005 5.4L means 4spd trans. 2 gears to accelerate, one to cruise. Hit a hill and you'll be down to 2nd again and 4000+rpm. That's life with what you got. I had a 2003 F-150 5.4L/3.73. Same deal. Let it spin! Your 3v engine makes its peak torque around 3500rpm.
If your truck is 2WD, you may consider changing the differential gearing to 4.10. That will really wake it up for about $500 installed. 4WD trucks at least double and the value isn't usually there.
