You can connect a 12v supply, capable of sustaining approx 8 amps, or a 110 ah battery via a 15A fuse, directly to the DC terminals of the fridge, or you could wire up a grey socket with pin 6 to (+) and pins 3 & 7 to (-). If you use the grey socket arrangement then you can power the fridge on pre and post 1999 caravans. There are no relays in the caravan that require to be bypassed, the fridge is fed directly from the Grey plug. ( on post 1999 vans the fridge supply operates the habitation relay, but this only effects the battery charge / permanent feed and not the fridge).
If you use a battery divide the ah by 8, take of about a third, and this would give you the approx. time a fully charged battery will be good enough to heat the fridge element.