I have towed a couple of loads that will push the scales of the combined load to just over 2 tonne. The Td5 Disco 2a auto does it well enough much better than the Defender 300 Tdi manual. I think the auto helps a fair bit as the torque converter allows the engine to spin up and the turbo to come on boost quicker than anything I can do with the clutch on the manual.
As for 5th gear towing with the R380, I would avoid it. You can cruise on the flat without doing too much damage but I would not load it up. I cooked the bearing and chewed the gears on the layshaft where the drive comes in off the main shaft gears. When the thing let go I was not towing just had a heavy load in the vehicle. Your manual will be able to handle the load better when the main shaft carries the load straight through the box. All makes have the same issue, bigger boxes can have bigger bearings and bigger radius gears resulting in reduced pressure on the cogs to transmit the same torque. Just keep it in mind and use 4th as soon as you start to drop revs under load in 5th.
