I don't think that driving distance on the highway is particularly related to wear caused by towing, if there is in fact any additional wear. Wear is usually related to high temperatures, which are tied more closely to start/stop cycles, and number of shifts (or the torque converter not being in lock up mode). If you are on the highway with the torque converter in lockup, there is no real cause of wear, towing or not.

There are lots of posts here by withidl relating to towing 8000+ pounds over the continental divide multiple times, in an early model 4.4. No transmission issues over the vehicle life.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue