Even trailer towing, per se, will not overheat a transmission. What overheats a transmission is slippage, pure and simple. When the torque converter is unlocked, and the input speed does not equal the output speed, the transmission is in a state of slip. This is normal and fine, but when in this state excessively, the fluid will get hot.

City driving and slow speed driving (stop and go) will do the most to heat up a transmission. Of course, towing can exaserbate this phenomenon through increased load on the engine and transmission. But in general, towing won't necessarily add more heat to the transmission, especially over long distances on the highway. As long as that torque converter is locked up, the transmission will stay relatively cool.