They have discoloured due to the manufacturer using unsuitable and/or inferior materials, the discolouration is a layer of UV degraded plastic, mild cases can be removed with Tcut or similar, alternatively fine wet n dry will be needed to remove the top layer of degradation. We had to start with medium wet n dry to get ours back to white on a 5 yr old caravan.
Having spent 15 yrs in plastics research I can conclude that caravan manufacturers use the most cheap, nasty and unsuitable materials possible, materials that would look better for longer are available at negligible extra cost but fewer people would buy new caravans if their 10 yr old vans were still looking good and dry.
