Camper van fridges work in a different way to the fridges in your kitchen. I'd always thought that it was odd that you could run a fridge by burning gas to heat it. In fact, what you are doing is heating a solution of ammonia in water. The ammonia has a lower boiling point and turns into a gas. Further round the pipe work, it cools and condenses back into a liquid. This liquid is then sent into an evaporator inside the fridge, mixed with hydrogen gas. The food and air inside the fridge gives thermal energy to the ammonia liquid. The fastest (hottest) particles can escape from the liquid, leaving the slower (colder) particles behind. The fact that thermal energy from the contents of the fridge has been used to do this makes the stuff in the fridge colder. I think the gas heating at the beginning is just to maintain the flow of ammonia around the system. It is the evaporation inside the fridge that does the cooling. (Note to Year 10: don't copy all of this - choose the bits which actually answer the question)
Notice the metal fins around the part where the ammonia evaporates (inside pipes!). The metal fins increase the surface area touching the air. Notice also that it is at the top of the fridge so that when the air loses thermal energy, it cools, shrinks, becomes more dense and sinks over the food, chilling it.
And thanks to Mrs B for pointing all of this out to me.
