Nearly all refrigerators and freezers are sealed systems. And they will loose their freon over time. Some don't .
The coils are in the bottom inside the freezer behind the back wall cover. The fan is also there but you know it's running.
You have a auto defrost system that may have failed. check the coils.
check defrost timer, defrost heater, defrost thermostat
Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com
Your defrost system parts are shown here.
http://www.searspartsdirect.com/partsdirect/getSubComp.pd?modelNumber=2539284215&productCatego ryId=0142000&brandId=0582&modelName=UPRIGHT-FREEZER&diagramPageId=00003&componentDescription=S YSTEM&documentId=P0102002&backToLink=Return%20to%2 0Sub%20Components%20list