They are required to be painted a light color that will reflect the sun's heat. When starting out on a trip your refrigerator should be run, to get it cold, before you place any food in it. This means the mechanism does not have to cool the unit and the food all at the same time. A good airflow over the tubes and pipes at the rear of the refrigerator is a must. Don't store anything inside the service door that will impede the free flow of air.
Heat rising from the condenser tubes at the top of the unit exits through the roof vent.
It is a good idea to look up inside this area to make sure this vent is clear. Birds have been known to build nests in this area. Sometimes the roof vent gets squashed when people are walking on the roof. The refrigerator also depends on circulation of air inside to allow the cold air to drop to the bottom and force warmer air to rise to the finned cooling surfaces. Try not to stuff the refrigerator so full that the air cannot move.