I did about 7500 miles towing trailers round europe last year behind a SWB
toyota land cruiser tdi and a VW passat TDi130 estate.
With the land cruiser you didnt notice you had anything on the back (even
with a Nimbus 4 in a cobra trailer). However fuel consumption was in the
low 20mpg and it was uncomfortable.

The Passat was great. Measured fuel consumption was 47mpg with a LS6/18 in
a tube trailer. Very comfortable and stable up to well in excess of the
legal lmit.

As said elsewhere the TDI engine characteristics are great for towing.
Loads of mid range torque that avoids constant gear changing that you get
with a similar size petrol engine.

The trick with stability with trailers is to get the weight distibution
corect. You want as much as possible over the axle(s) but with a sensible
amount on the draw bar. (not hundreds of pounds as I have seen advocated
here) If you put all the spare gubbins in the front and back of the trailer
you end up with a "dumbell" which is potentially unstable.

Nigel