Have noticed the same problem with a mkII using two dirrerent caravans all I can say is dont load up the van too much and try distributing the load differently. At the end of the day if 60mph is too much and 55mph feels safe then you have your answer. A good tip when you see someone approaching you to overtake is move out slightly to the right making them take a wider berth.Then move left just before they reach you creating a wider gap between you a you will feel less drag etc.