These are good fridges (I have one), but this is a common problem (I have had it).
When the light comes on are you able to check the voltage at the battery/fridge with a multimeter or does your vehicle have a battery level meter installed?
When the fridge is first turned on it uses more power. If you are running the engine (i.e. driving) there will be more power as the engine will provide lots more than the leisure battey.