OK hows this work then??

Bit perplexed here. I have been towing my caravan (foldy up job so admittedly not much of an aerodynamic issue) all last week and I have found that I get exactly the same fuel economy with the van on and with the van off the car?? Im not complaining but I would have expected it to drop???

In my opinion this proves that the car has plenty of extra power left under both conditions, with or without the van attached. That it has not to be pushed at all to tow and that the engine can 'work' the same in both situations ?

Your "foldy" van is obviously not heavy enough,or causing extra drag by being bulky to cause any problem to the motor, so I wouldn't worry, you seem to have got it dead right, happy caravaning.