Your requirement is to keep a large fish fresh for a few days?
By definition not strictly possible, but if you are indeed out in the sticks, I'd forget 12v only options unless you have shares in a leisure battery business.
There are "natural" options involving running water or digging holes/shelter etc but an easy option is to use a portable 3way fridge
(=mains/12v/gas).
When travelling, run it off your car.
When stopped eg to fish, put it in the shade and run it off a large size 907 camping gaz bottle - should run best part of a week off a full one.
When at a hotel etc run it off mains
Check out brands such a Waeco:
http://www.mobilegas.co.uk/mobilecoo.../combicool.htm
I have one for camping and never looked back.