It is unusual for the fridge supply on the caravan from the 12s socket to have a fuse, if it will be fused anywhere it will be on the car side, where depends on car manufacturer and or if the tow bar is fitted or aftermarket, the most common faults with the fridges on 12v supply is a bad earth, have you actually checked this, also if you have a multimeter check for resistance across the terminals 6 & 7 on the caravan as this will check to see if the heater element in the fridge is working or burnt out, they have2 elements in, one for 12v and one for 230v if you have a test meter or lamp also start the car and between pins 6 & 7 on the car, this should either light the test lamp up or indicate around 14.4volts on a multimeter.