Long trip = continuous low throttle position + high vacuum = low power = no heat build-up.
If your ATF temp hasn't stabilized in 5 miles something is wrong.
Where to measure AFT temp: at its hottest point - the line out of the transmission going to the cooler. I've seen gauges plumbed into the return line, which tells you the cooler is working but nothing else.
You and I differ here.. I want to know what the temp
is in the pan.. thats the fluid waiting to go into
the trans itself.. on outlet temps.. thats jumping all
over to an extent