I have a Maytag side by side fridge that is 8 years old.
The freezer thermometer reads -15 degrees and the fridge temp is 55 degrees.
The compressor is always running, but I do hear a fan in the freezer side with the door open. I assume there are actually 2 fans and not just one??
I pulled the crisper drawers and the return air flapper is not stuck or frozen closed.
From the FAQ list; it sounds like the evaporator fun, but how do I tell for sure??

It sounds like your defrost thermostat is defective and the 15 degrees side is freezing up blocking the circulation of air to the fridge part