Hello Gordon,
Does it always happen with the same appliance, or can it happen with several different appliances.
If it is just one appliance every time, then I would suspect the appliance is faulty,
But if it can happen with any appliance, then it throws suspicion on the MCB its self, or possibly the caravan wiring.
Please remember that mains power can be lethal so for your safety and insurance, the fault must be repaired by a comment person working to the current wiring regs.