Jason -
You are asking good questions. In my experience (and I hope others will chime in), the answers are not obvious and in some cases they are even counter-intuitive.
For example, "If I slow down, will it lower the temperature?" You would think so, since less power is being transmitted through the transmission. But if you slow down, there is less air flowing through the transmission cooler, so less cooling is happening, and the situation may in fact get worse. On my last trip, going over one particular pass, I found that there was a broad "sweet spot" at around 40 mph. It wasn't very pronounced, but it did seem that going slower than 40 actually caused the temp to rise - and struggling to go faster than 50 also caused the temperature to rise. But I'm sure this was true only for the particular load and the particular grade and altitude that I was encountering at the moment. Different conditions would have changed the answer.
Stopping entirely should help. Put the transmission in Park and leave the engine idling so that the fan will blow air through the radiator and the cooler.
Manual downshifting to a lower gear may help.
Yes, a cool day will result in a cooler transmission, since the cooler air will remove more heat. The worst condition is obviously high altitude (thinner air doesn't remove as much heat), steep grade (where the tow vehicle is struggling), and a hot day. Relax any of those demands, and you'll do better.
As Lars points out, don't use overdrive except on very flat terrain, and don't let the engine RPM drop too far. Either one will let the torque converter slip, which is the primary source of heat in a transmission. On the other hand, my personal preference is not to let the engine scream up to high RPMs, either - it is hard on the engine, especially when it under heavy load.
Don't get too bogged down in the details of all this, and certainly don't get yourself over-stressed. All of this is best handled by simply observing what is going on, with your particular tow vehicle, your TM, and your load - and then changing something to see if it helps. The advantage of the gauge is that you can perform those experiments and see what happens - without a gauge, you're flying blind. And in the worst case, of course, the gauge keeps you from blithely descending into disaster.
Bill
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