Thanks for narrowing it down a bit...do you have things in the fridge? If so, make sure air can get around inside. A small fan inside will assist in making cooling more efficient. They have a battery powered one. Blocking the air flow will decrease the efficiency of the fridge. Let's do another test.
Unplug the thermister. You will find the plug up by the light inside the fridge. The light will be OFF as well. That's normal. Once unplugged, the fridge will run constantly, meaning the freezer will get cold first followed by the box. You should see temps dropping. Take temp readings every hour for four hours. Do not open the box between temp checks. If each reading is less than the preceeding one, the thermister is bad. Simple fix...buy another thermister, about $30. Unplug the old one and plug in the new one....Dennis