Surely putting a ZIG unit into the car to power it to 240 volts and then back again will be a bit of hassle, fair enough if you want to run other 240 powered items at a later date but I think ideally you would have to set up a split charge relay and battery in the boot so you dont kill the normal car battery off.
Dont forget a deep cycle battery is designed to drain over a long time but doesnt have the instant ommph to start a car. Where as a Car battery is designed to give instant starting power but doesn't like to be drained down too much as it takes away its charging and power storing capabilities.
