I fitted my last van with two small computer fans controlled by a thermostat. Considerably cheaper than nearly £50. It seemed to work okay in southern France until a noise sensitive neighbour said the fans disturbed them at night as they cut in and out. So I switched them off. They must have had far better hearing than me but I guess we were insulated by the fridge, cooker and cupboards and we slept at the rear of the van with bedroom door closed to keep dogs out.