An unlocked torque converter is generally what makes heat, not high RPM. Most of the new vehicles will keep the converter locked in T/H mode if you are at highway speed. If they need more power, they just downshift instead of unlocking the converter. No slip = no heat.
Also, the converter is usually unlocked at slower speeds, less than about 40MPH so that's also when heat can build quickly. Combine that with low airflow and that's when you can fry a trans.
I usually run 5th with my 6.2 SD if it's nearly flat. Otherwise, if there is a series of slight grades that's causing it to shift repeatedly, I'll limit it to 4th. If it's really irregular terrain, I'll just run manual mode.
