Is your fridge in a slideout? If so, in order to vent out the wall (as opposed to a fridge NOT in a slideout, vents through the roof) - the fan must come on to assist with hot air movement. If you have not heard the fan come on, go outside and check in the access panels.
On mine, the fan is mounted inside, just under the top opening. The thermal switch that operates the fan is mounted to the fins at the top. After verifying I had power, I bypassed the thermal switch and connected the wires together - the fan runs 24/7 now and its cooling like new!
My new switch should be in tomorrow at the nearest Dometic "authorized service center". If you have a DM2852 model fridge, the Dometic part number is: 3104133.016
I called Dometic myself and got the part number in order for the "authorized service center" to order it - he had no parts breakdown for this section of my fridge.
Having said all that...if your fridge is venting through the roof and not in a slideout, chances are - you don't have a fan. Must be something else.