I had no way to monitor the transmission coolant. I just checked the engine coolant.
The automatic transmission fluid is routed through the engine's radiator and is kept at the same temperature.
My notice that the coolant was at a lower temperature when towing in low range as compared to high range, indicates two things:
a. lower transmission fluid temps due to less torque converter activity.
b. lower engine temps due to reduced load ( the 2.64:1 reduction in low range makes it easy for the engine to perform - like the low gears on your mountain bike make you sweat less )
Since the single most serious killer for automatics is heat, the lower temps will keep the automatic happy for a long time.
Mpg, according to the trip computer, did not differ much in high or low range.
Towing up a 11% grade I registered 5.4 mpg in low range and 6.0 mpg in high range.
Using low range brings down mpg and rpm will be up. However, the ML's engine is like a race horse: it loves to run! I can't say that about the J GC's engine.