What are the weights of the car and caravan? You need to look at the kerbweight, max tow weight, gross train weight and max loaded weight of the car and the max weight (MTPLM) of the caravan. Come back with the weights and we'll be able to help.
According to Parkers the 2006 A4 2.0 diesel weighs around 1500kg so the 85% weight would be under 1300kg. How heavy is the Challenger? I think you need to look at a lighter caravan.