Gelcoat is the outer layer on fibreglass caravans and boats, and sunlight breaks it down over time. The surface goes chalky and loses its gloss, and on a white caravan you'll often find colour transferring onto your hand when you run it across the panel. Washing won't fix oxidisation because the damage is in the surface rather than on it. Restoring oxidised gelcoat means cutting it back to fresh material and then sealing it, which is why a neglected vehicle costs more to detail than one that's been kept covered.