I see a lot of threads and questions about this, so figured I would share my real world data...
I have a brand new ARB Fridge, and a brand new Sears DieHard Platinum 31M Marine deep cycle battery. Bought both based on reviews, comments, etc. on here and on MUD (I have a Land Cruiser). Started the dual battery install the other night, but had not yet finished all the wiring. The weather cleared here and decided to do a one night camping excursion. So I did a beta test of the set-up....
Loaded the fridge to about 75% of capacity with a variety of food and drinks. Set the target temp to 31 degrees. Used 120V at home to get it down to temp -- worked great, was pretty quick to get cold, did not measure this part.
Put the fridge and the battery in the Land Cruiser at 1pm on Wednesday - nothing but the fridge connected. Measured voltage at the posts - 13.33
Monitored it at the posts over the evening, and the in morning, and all the way to back home...at conveniently - 1pm! Voltage then - 12.66
Ambient temperature was near 70 during the day, maybe 40 overnight up in the hills.
The battery was NOT connected to the truck wiring, just the ARB fridge -- so it was only being drawn down, nothing adding to it. Just thought I'd share the real world data for fun.