When towing my TT in the high and steep mountains, I decide what gear to be in by watching the tranny temp. If the temps starts to climb I drop a gear or two to bring the rpm's up and the tranny temp down. It works for me.
When going downhill, I shift early and let the engine do what it can to slow me. I still have to apply the brakes from time to time because my turbo gas engine is not great at motor braking. Small engine, big horsepower and torque at low rpm's