No, you'd better get at least a Ford F350. (sarcasm)
Don't be afraid of the engine "working hard", it's designed to run at any load for an unlimited amount of time. As long as you don't overheat the engine or transmission, nothing bad will happen. You can tow up mountain passes at full throttle for as long as you want as long as the temps stay normal. On flat roads you might glance at engine temp periodically and don't worry about transmission temp or add a air cooler. (It's already cooled by coolant in the radiator.) If you are racing up mountains with a trailer you might consider getting a scangauge or something similar to monitor transmission temperature.
I would probably leave the transmission in 5th on flat highways unless it's hunting for gears. I can't remember if shifting it to 4 forces the torque converter to stay unlocked, but if it does that generates more heat. Anyone know? Maybe it says here
http://www.ezdries.net/Vidpics/temp/A750F_Automatic_Transmission.pdf or in the service manual?