Waeco Fridge Performance

I have recently purchased a Waeco FF-30 (30 litre) fridge ($599) and ran the following test over Easter. I completely filled the fridge and left it on full freeze overnight. When we arrived at our campsite the next afternoon the fridge temperature was -16 deg C. I immediatley turned it to its warmest setting. It ran for 2 days on a 4 year old Exide ED5 75 amp hour battery and remained at around -1 deg C. The 10.5 volt cutout then shut the fridge down. I turned it off and for the next 2 days the temperature remained near zero degrees C. It was opened twice to remove frozen meat. On the third day the temperature started to climb rapidly. Upon opening the fridge two bottle of water were still half frozen. Ambient temperatures were in the mid 20s during the day and low teens at night.
A friend of mine performed a similar test on a similar fridge at the same time and achieved similar results but his new battery was still above 10.5 volts after 3 days. Both fridges were in insulated bags in the vehicle.