Refirgerator Freezer Frosting

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The freezer in our 2004 2720SL frosts up. There is frost and even ice forming on the inside walls, ceiling and lip. This takes up room that I could use for food. The only solution I have found is to defrost it with a hairdryer. We have the refrigerator running on electric (shore power). Does this happen to anyone else? How can I avoid this problem? Thank you in advance.