I've got just about 50,000 miles on my D/A.
At about 25,000 miles I did a flush and changed to Transynd

95% of my use is towing the toyhauler.

What would be the common sense mileage to change
the Transynd fluid with mainly towing use.

Heavy duty use is 75,000 miles off the top of my head. Up to 125,000 with regular use. Split the difference and change it in 75,000 miles.

I wouldn't let any fluid go past 50K.

Even the best fluid is cheap by comparison. 75k would be tops, but when towing etc, 50 is plenty.

Here you go, right from Allison's literature. 75K filter changes and 150K fluid changes for general duty. 75K for severe duty, and that is on a 3000 series that takes much more abuse than a 1000 in a light duty truck.

http://www.hotboatpics.com/pics/data/500/23transynd.jpg

I plan on running mine well over 75K. Then again I have a lifetime warranty on my Allison. ;)