If the freezer door seal is bad, you will see a line of frost/ice from the door to the rear of the back wall. If you only have ice build up on the back wall, that is normal. These are not self defrosting refers in RV's. Depending on the Humidity and how often you open the door depends on how much frost/ice build up there is. The usual test for a gasket is to insert a dollar bill all around the door gasket close the door and see if there is resistance when pulling it out. Yes, there are no gaskets available, only doors that come with the gasket. Doug