OFF OFF OFF.
"Overdrive" is just that... here's how it works: it makes the wheels turn more than one rotation for every rotation of the transmission/ engine. So basically when you're in overdrive your wheels are spinning faster than your engine, which takes stress off the engine in normal freeway driving when the accelerated mass of your car can maintain your speed and your engine doesn't need to work as hard. But if you are towing more weight, going uphill, etc., it actually makes your engine work HARDER because it has to catch up to the wheels to push that load. It's like this: imagine (and these are just arbitrary numbers) for every 1 rotation of your engine, your wheels are spinning twice. In normal freeway driving, the mass of your car at speed helps to push itself and the engine really only needs that single rotation to every double rotation of the wheels to maintain speed. But if you add weight, you are asking your vehicle to pull up to twice its mass at half engine power. This will make your vehicle sluggish and tear up your transmission, because it's trying to convert the engine's power to wheel power at a lower rate. Your engine will bog down and have to work twice as hard to maintain the same rate of speed.
When you turn overdrive off, it sounds/ looks like your engine is working "harder" but it's really just working normally. Towing is hard on your vehicle anyway, but you are NOT doing it any favors by keeping it in overdrive, you are actually damaging more than you are helping.