How long can I run a car fridge with dual batteries?
The key on fridge life will be on what setting you run, and how efficient the fridge is.
On the freezer setting, the A series Engle fridges (an old model - worst case scenario) will draw 5.2 amps per hour until the required temperature is reached, then they will draw 4.8 amps per hour, every hour.
Given 20 hours running you would expect to use about 100 amp hours from you battery, even with a 120 amp hour battery, you are not in business for more than a day without a charge.
However if you use a lower setting it will not only draw a lower current when it reaches the desired temperature, it will also cycle, giving the battery periods when no current draw is occurring. Once again using the A series Engle's , depending on the setting you use, it may use only 1.5 amps per hour and cycle on only 20% of the time - drastically improving the "life" of your battery per charge.
It all depends on what you have in the fridge - if its just food and stuff, then you could survive on a lower setting. However if its beer, then just above 1 or 2 degrees C (cold but not frozen) is a must - sacrifices are just unavoidable sometimes ;-).