I don't think your 285/65R18's are significantly large enough to require re-gearing, but taller tires will lower RPMs and cause the torque converter to work harder, which can raise the fluid temps a bit.
Pan temp on mine seems to run 196-200F when towing in 4th gear. Outdoor temp (60-90F) seems to have little effect. Highest I've seen was ~203F when pulling through some of the mountains in Wyoming over the summer. Torque converter temp runs hotter, particuarly when the TC isn't locked under load or when it's not in 4th gear. I've noticed when cruising but not towing that the pan temps run around 200F in 6th gear, maybe slightly higher a couple degrees higher depending on altitude and outdoor temp. They'll climb up to 210F+ if you're heavy on the gas pedal since the TC will generate heat the harder it works, especially if unlocked.
FWIW there's a "warmer" on the AT line which is designed to get the fluid temp up to operating temp quicker to improve mileage. I don't think it's possible to get the temp below ~195F unless you remove that. That said I'm pretty sure Toyota engineers know what they're doing and Toyota ATF WS is designed to run at the warmer 195-200F temps all day long.