Hello Anthony,
As with cars, differnt makes and models will depreciate faster than others.
Again as with any product bought new, as soon as the ink has dried on the contract, it looses 17.5% from the Vat element. As for other factors, then age, wear and damage, then there is the general state of the caravan market, which with climbing fuel prices and VED tax, tolls and congestion charging on the larger cars needed to tow even modest caravans, it is likley the market will be depressed as fewer peopel switch to caravaning either new or secondhand.