hi Jim, same as above, I find it better towing in fourth, both in fuel consumption and responsiveness of the unit, solo the car is in top most of the time, so it can be difficult not to automaticly change up while towing as you think it is better for the engine.
however sometimes it is easy to forget you have over a ton attached to the back when rolling along the motorway at a constant speed, most modern diesels have the most torque around 2800rpm with a difference in gears of about 300rpm @ 60mph keeping the rev range within this torque range actually saves fuel in the long run.
give it a try next time you are out with the car, do a constant 60mph and see which gear is best for this rev range you will then have a answer