A 100Ah 12 volt deep-cycle battery can run a 400W  DC load for 1.5 hours at the recommended 50% Depth of Discharge.

Hours left in battery = battery capacity in Ah / current draw in amps

Yes. They are naturally deep-cycle and can be discharged up to 95%.

Running time in hours for 400W fridge fed from 100Ah battery = 600Wh / 40W = 15 hours

Battery watt-hours = amp-hours x voltage