Some folk (like me) have also fitted a small 12v PC style fan on the top vent to help expel the air during hot times.
Just like your fridge at home, the MH one relies on the ambient temperature being between set points to allow the fridge to function correctly. However your fridge at home is unlikely to be find itself being parked in the alps one trip and then some nice hot spanish resort the next.
Thus if the ambient temp is too hot or too cold you need to take extra steps. Hence covers for cold weather and maybe a fan for real hot weather.