Its heat that kills the trans.
Most people dont realize that their Tow Vehicle has a lock up torque converter. This means that under a certain load, the TC will unlock to give you more power, but at the same time it creates way more heat while unlocked.....think one fan blowing trans fluid on another fan (basic torque converter).
If you dont have a trans fluid temp gauge, then you really have no idea what is going on. Going up a big hill unlocked you may see temps as high as 230+ which is bad, some higher im sure. This will then cook the seals and make them hard. Then the seals get blow by and the clutches in the trans have no holding power, hence slip.