Hi Tom,
Zaffiral has pointed out the dynamic effect of adding mass to teh extreme ends of a caravan, but there is also the practical aspect that the caravan may not be constructed to have that amount of extra weight fixed to the rear wall.
The practice is quite common on the continent, but there are two significant differences that may apply. Firstly the continental caravan tend to have longer A frames, which tend to be less prone to instability issues, and secondly most continental vans have addition strengthening members in the back wall to accommodate the extra weight.
Some UK manufactures do offer rear wall bike rack fixings, check with your supplier.