Hi,
It is a fairly simple task to get a 12v supply from your car. On the 13 pin from the car should be a 12v permanent live. In most modern caravan you can switch between the power coming from the caravanbattery or car battery. If you make up a lead with a 13 pin plug and socket you can pick up the permanent 12v power from the car and send to the caravan. Of course you have to be aware that in time you could discharge your car battery. I have had this setup for many years and check the car every couple of days to ensure the battery has still enough power to start the car.