You can disconnect the thermostat but you willl have a freezer that runs constantly if you do. Ammonia type refrigerators are notoriously weak and at their worst when empty. I suspect the heat of the day is raising the temp in the fridge. Take a couple of gallons of water and place those in the fridge and see if it holds temperature better. If all you have is air in the frig when you open the door the inside temp will drop drastically. The water will act as a heat sink and help maintain the temperature.