Cover them with something. A good quality wax for instance. Look out for the ones with petroleum distillates as this can dry out the vinyl and cause them to crack and peel before they ever get old enough to fade.
Many have been using 303 to protect. It offers a good UV barrier. A friend that works in vinyl graphics told me that it is one of the better products. He said that the RV graphics are generally the cheapest and weakest type of vinyl so care is important. I am planning to have his company redo the front of mine with something better.
2006 Ram 3500 Megacab Cummins
2012 Keystone Sprinter 311BHS