Every time you open the door, moisture laden air rushes in. Shut the door, the moisture goes to the wall and freezes. Or if seal isn't tight all the way around, ice might build up at the entry point for air around the gasket. Higher the humidity, more ice build up in freezer.

In addition to what Barbara said - most/many RV refrigerators do not have automatic defrosting for the freezer but do for the refrigerator part. We had to defrost ours every couple of months in dry places like AZ - more often in places with high humidity.