If the frig is the only thing running the longest i have left the batteries go is 3 days, that took them down to 10.8 volts. If i am parked I will run the truck a few times each day to charge them off the Van alternator. I have never run them dead and that is NOT a good thing to do, you will kill them and they will not hold a charge once you drain them past a certain voltage.
The reason i went with the golf cart batteries was because of the way I use them, i have more electric stuff them most and i use alot of it at the same time, which drains them faster and i recharhe them more often. That cycle is not the best in any batteries but the design of the golf cart battery lets it work better. I am sure turtle can tell you way, i have no idea, all i know is that they work better then the 2 12 volt deep cycle marine batteries by deka i had that i killed by letting them go dead......after that they would not hold a full charge long enough to watch a 2 hour dvd .....