Hi Neil,

Another alternative is to run it off the normal battery during the day and off one of those battery packs overnight. The battery pack can be used to jump start the vehicle if you have to and you just charge it back up while travelling off the 12 volt sockets. I bought a reasonable one from Super Cheap for $149. Not as strong as the ones from a couple of the 4wd shops but a lot cheaper.

I have one of the three way fridges so I rigged some heavy duty cable to the back with a 16 amp fuse up near the battery and put a relay in a box with an accesory plug on top. I then put a cigarette lighter plug on the relay side so when I plug into the socket in the back and turn the ignition on the accessory socket comes live. Sincethe three way fridges are very inefficient on 12 volt I plan to take it out of the back and run it on gas when we stop. But with the relay in there I can't run the battery flat unless I leave the ignition on.

You could probably do something similar but run on the jump start pack at night.

I did price the dual battery systems from Piranha and Rotronics to use with my boat battery but they were going to cost a packet and I still couldn't run the three way fridge for more than a few hours off that.

Not exactly what you were asking but hope it helps.

Tim.