A fan blowing a small amount of air upwards past the cooling coils on the rear of the refrigerator can work wonders.
In cool weather, the heat from the coils sets up a chimney affect, drawing in cooler air from the bottom vent while the warmer air rises and carries away the refrigerator's waste heat through the roof vent.
In hot weather, there isn't enough difference between the ambient air temperature and the temperature of the refrigerator's cooling coils to set up the convection flow, so the air becomes stagnant and can't carry away the refrigerator's heat, bringing the cooling process to a halt.
All it takes is a little bit of airflow across the coils to restore the refrigerator's cooling. You can use a small 120 volt clip-on fan attached to the lip of the lower vent and set to blow air upwards across the coils to test the concept, then replace it with one or two 12 volt muffin fans for a more permanent installation.