Icing can only happen from the Humidity in the air inside the refer. Icing is usually normal. The fins will be 10 degrees COLDER than the inside temp of the items in the refer. HOW thick is the ice? Norcolds have a built in defrost feature for this type problem. The feature ONLY works when the refer stays ON continuously for more than 60 hours. Every 48 to 54 hours, the Norcold refer shuts OFF for 2 to 3 hours. This allows the fins to warm up and the ice to melt and drain outside to the drip cup. If you turn the refer OFF and then ON during that 60 hours, the timer starts over again. You could have the evap pan drain tube check valve not installed and that allows air back up to the evap fins. You can have a bad seal on the refer door allowing air inside. I have also had refers where the customer installed items in the refer that pushed against the closed doors and this caused the door to have a slight gap that went past the door seal. Doug