Black streaks, especially on the roof edges and under the windows of your vehicle, are called rain streaks. These streaks are a mixture of vehicle dirt and sealant that then flows down the vehicle through rainwater. These stripes are nothing to worry about and are completely normal.
You can effectively remove rain streaks with the right cleaning products, which you can obtain from your HYMER dealer or specialist retailer. When choosing a cleaner, always pay attention to its compatibility with your vehicle.
You can find further tips & tricks for cleaning your HYMER vehicle here: Tips & Tricks: Clean your motorhome properly