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The fan does not have to be running all the time, in fact, you may not need to run it at all. The fan is temperature controlled and does not run all the time anyway. When most needed is when the fridge is being initially cooled, the weather is hot, and when the fridge side of the trailer is sitting in full sunlight. It definitely helps the system work easier but if the temperature is staying at a safe level without it why run it. If you are boondocking you probably won't want to run it just to save your battery. We seldom ever run our fan and the fridge does just fine. It's noisy and will drive you nuts after a while.
I recommend you get one of the wireless indoor/outdoor thermometers and place the sensor in the fridge to keep a monitor on temperature.