If this is the case the onus falls on you the customer to understand what your fridge draws (draws means what amperage does it use depending on settings) is it set on 2 degree or freezer , how large is the fridge and so on.
The average amp per hour draw spec can be provided by the fridge manual – for example a 50 litre fridge set at minus 15 depending on make , model and Ambient temp could use 3-4amp an hour continuous. If that was the case, the nomad has 100ah or amp hour at 80 percent dod, so you have 80 ah available, so your fridge drawing 3 amp an hour for 24 hours would use 72amp, and the fridge might cut out at 10.6 so you get 18 hours of power.
This is not a fault of the battery or the fridge. If you had a 200ah battery typical style battery you would still only have 100ah (50% dod) You need to ensure you are putting in the charge rate to counter the output.
This is the simple mathematical equation to how long your fridge will run. You must understand this basic example or you will expect something that is simply not possible