Hi Skye740,
I know mikealder linked to a split charge system, but what I think this will give you what
http://www.towsure.com/product/SelfS...bination_Relay
which will need some wire with it but a little cheaper than the other solution. These drive Caravan's fridge and charge the battery with a basic direct feed via 30amp fuse from Car's Battery Pos terminal and a good earth. The two wires to battery you wish to charge. A switch to isolate the battery connections when not attached would help to make the free ends safe.
DC850