When you see freezing on the back panel of most any freezer it is an indication that one of the three components of the defrost system has failed and causing your refrigerator to miss the defrost cycle. The=>
2304099 Defrost Control BoardYou can defrost the evaporator after removing the rear panel of the freezer with a hair dryer. Here is a link to the=>
RT18DKXKQ Roper Repair Part List You will need Adobe Acrobat Reader to view these documents. The Adobe Acrobat Reader is available as a free download. Click HERE=>
Get Adobe Acrobat Reader if you do not already have the Adobe Reader. Unplug the refrigerator and remove the control panel inside the fresh food section just above the top shelf you will be able to see the Adaptive Defrost Control Board and sometimes but not always you will see burn marks or signs of heat damage to the control board. In most cases this board will need to be replaced to get the refrigerator to defrost again. The other two parts to check will be the=>
2315531 Defrost Heater Assembly and the=>
4387503 Defrost Thermostat You can test the defrost heater using the short tutorial on the same page as the part. After testing the defrost heater and having a look at the defrost control board I can help you test the defrost thermostat which must be very cold to test with your OHM Meter in order to get the correct readings, Just let me know, Thanks
Sea Breeze