What make of fridge is it? If a Dometic or Norcold, those are different than a home (residential) fridge. They are an "absorption" fridge.
This type boils an ammonia/hydrogen mix and that produces the cooling. The ammonia/hydrogen can be heated to a boil either by propane or 120 volts AC house power, such as that supplied by a generator or shore power.
Our Norcold 1210 four-door requires about 450 watts of 120 VAC power, or about 4 amps of power. To produce that same amount of power using 12 volts and an inverter would pull over 40 amps out of the batteries.
With six batteries, presumably six 6-volt golf cart-type batteries, you may be able to have 40 amps pulled out but they won't last for days or anything close.
That's why people will run absorption fridges using 12 volts and the propane capability if shore power or a generator are not available. These types of fridges really use very little propane, nothing like the amount of propane that a furnace will use.
2020 Georgetown GT5 34H5