I am new to caravanning and have the opportunity to make use of a new Swift challenger 4 berth 520. I have a 1.8 Ford Cmax and the owner of the van has a 2.2 Vectra CDX, both petrol. From what I read about the 85% rule, neither of us can tow this van, based on their kerbweights. Is this correct ? Any advice welcome as it is all new to me !