Still needs to not use in on gas, just in case.I suspect Tony does not have hook up, he said he was going to Granada rally, and also mentions generator
The vents should have two sliding clips, look from below.Pop the fridge vent off on the side of the caravan and see if you can see anything burning or melting. There is a slim chance that a wire or similar has moved and is touching something hot, however as others have said, getting the fridge checked/serviced would be a wise precaution as the sort of smell you describe sounds like burning plastic of some sort.