Pick up power in the back of the fridge at the 12 volt or any 12 power. You can run wires through the thermistor hole to the back of the fridge and switch it outside or get fancy and run wires back over the top of the fridge and inside to a switch location.
Remember to unplug the trailer and turn off battery switch or disconnect battery before doing and electrical. 12 volt is just a positive and a negative wire so fairly simple. Install a rocker switch in the positive line somewhere to control the fan and I would add a 3 amp fuse in the power line also.
Good wire for wiring small 12 volt items.