Roughly a $1000 for parts and another $2000 for a professional to install. This would be good quality parts. Installer must be qualified and be insured against his errors for maybe 3 years.
It maybe less expensive to install a power management system.
Outside fridge? Is this one in a compartment or stand alone?
new Inverter no, converter depends on where it is and whether it is interval to the power distribution panel?
Big cost is new power distribution center including 120 volt and 12 volt systems. New shore power cord, and wiring.