I'm not sure why some owners manual state to turn it off wheneve you tow, and others advise you to leave it in OD, but turn it off if the trans is "hunting". I like the later approach, in general.
We just bought our '06 Mounty, and we plan to trade in our 3000 lbs popup for a hybrid travel trailer soon (5000-5500 lbs range, loaded weight). The biggest thing that ticks me off is that you can't select 5th gear. Hitting the OD button locks out 5th and 6th. If I could hold 5th, i would lock out 6th whenever I towed. But I don't like the idea of making the engine spin in 4th whenever I'm towing. Around town? Sure, I'll keep the OD off. But once I get on the highway, I expect to use OD unless it's hilly and the trans is jumping around a lot.
Another tip:
Using cruise control while towing will have the trans shifting more than is needed. When towing, unless it's absolutely flat, I keep the cruise off, and use a steady foot. When you see a hill coming, pick up a little extra speed, but not enough to downshift or unlock the convertor. Keep your foot at the same place uphill. If you lose 5mph by the top of the hill, who cares? Half-way up, if you have lost too much speed, lock-out OD and finish the grade. But if you can keep the trans in 6th, and not lugging, that's the goal, and you will get the best fuel economy.