I think I run a bit cooler, in the 190 range.... the last trip you and I were on, climbing up the east side to Cerro Gordo, and climbing up to Lee Flat I was hitting near 220, the gauge did go up, but didn't go past 3/4. I think I was low on coolant at the time which probably contributed to it. I could hear my coolant boiling in the radiator.
When I did a trip out to WY and back last summer pulling a small pop-up camper, going up some of the big inclines, when the engine would be 4k RPM to keep me moving up the hill, I would see the temp gauge go up... but it goes up to a certain point and stops there... as soon as the RPMs come down, so does the temp.
I do like the idea of adding a 2nd trans cooler... I find that if I'm motoring around in the mountains, if I don't shift to "L" my trans temp warning light will come on. because at low speeds the friction in the torque converter heats up the fluid... shifting to L seems to lower the stall speed on the converter causing less slip which creates less friction which creates less heat.
Adding an extra cooler + an electrically controlled fan seems like a big (and fairly easy) win for low speed desert driving