By the way, my setup is a 4 wheel trailer thats about 1000 pounds that has surge brakes. (Ps, its a home built with cavilier rear hub/brake assemblies bolted to square tube steel frame.) For towing, what I have always done is leave it in drive (4th gear) as long as it would lock up. I will shift it to 3rd if either its a long hill or in the rare case that there is a 20 mph headwind and at 70 mph it will not stay locked up in 4th.

If you keep it locked up in 4th, its pulling about 2000 rpm in 4th, and that helps the economy plus wear and tear vs lockup in 3rd which pulls nearly 3000 rpm. I assume the reason they caution people to tow in 3rd is that if the transmission unlocks in 4th for long periods, there is lots of hydraulic inefficiency and there is not enough heat rejection in the transmission to keep it cool. I figure that as long as the torque converter is locked, thats not an issue. My previous '93 3.8 transport went from 110,000 to 180,000 miles mostly towing and never had a transmission issue using this strategy.