I've got an older 200tdi and regularly tow our Trak Shak camper trailer (1.5 ton) on and off-road. It does the job easily enough, although head winds, inclines etc will slow it down.

I use 5th when I tow however I will say that if the engine starts to labour slightly (you'll feel it through the accelerator) then I change back to 4th straight away. Also any hills or head winds will see me back in 4th straight away as well.

Basically don't labour the vehicle at all when in top gear, as it's not designed to have load through it.


Cheers
Chris