I think we might need to go back a bit...
Joe - you're saying that the back of the car is too low with the caravan AND four people in the back, right?
What about when there's just four people, no caravan?
The reason I ask is that I think you might have too much weight on the front of the caravan. In English, we call it "nose weight" - the amount of weight pushing down on the tow bar. Have a look in a local towing/caravan shop - you can buy a special device to measure it.
...or just get a cheap set of bathroom scales, and put the front wheel of the caravan on them...
There has to be a bit of weight, but not too much. 50kg is the maximum rated amount for the 2cv towbar we've got, if I remember right. If there's too much, there'll not only be too much weight on the car's suspension, but too much load on the towbar. Too little, and the caravan won't be stable, and might "snake".
With the caravan on, you can move some of the stuff from the boot of the car to the caravan, to help remove some weight from the back of the car. It should be easy to make the back of the car lighter WITH caravan than without.
