I'd replace it, if it's breaking down its only going to get worst.

Strange that they put Polystyrene on something like that in full and constant sunlight , which is styrenes nemesis,  as it degrades the outer layer . The harsh Aust climate will kill most things that are built for european conditions . Water based paints wont hurt PS , but it may have been better to paint it prior to the degrading starting . I'd go back to Dometic and speak to them about the issue , asking for it to be rectified at no cost to you .