You don't say under what conditions the dealer measured the 50kg (presumably the caravan was unladen). It is a rather low figure that won't help the stability of the outfit. It's always best to make full use of the limits available (i.e. the lower of the two specified for car and caravan, respectively). I don't know what these limits are but I cannot imagine that they are less than 75kg in either case, so it would be best to put appropriate ballast in the caravan to achieve this figure.