Ring do a 12 v extension/adaptor to allow more than one 12v appliance to be run off a single socket. This has a 3 leds such when the green is lit the cars charging system is coping with the load ok, amber is a warning that you are drawing too much power and effectively discharging the car battery, and a red one indicating satisfactory connection.
I thought manufacturers advise against using coolers in the boot, due to the lack of air circulation/cooling? We have a Waeco CAB 40 cooler, and I tried this within the car on 12v, and the heat output from the cooling fins is really significant-the current draw is also well over 6 amps. As mentioned above this is on top of the caravan fridge, and things like satnav, kids dvd player, and so on.