This is an ongoing problem, and I am sure it is me. We started with a cab over camper, went to a 5er, and now have a 29' Type B+. All were new when we bought them. In each case, I have had to defrost...
I don't think it is a refrigerator performance issue. You are just using it where the air is too moist, the amount of frost in normal for where you are.
I remember how quickly frost would build up in home refrigerators, before they built in the warming cycles and hid the cooling coils so we would not see what was happening.