So, you are correct in saying that you can set up a DC-DC charging system by connecting it to your car battery, however, there are caveats to this.  You will require a specific DC-DC charger to be added to your electrical setup to properly charge your battery and prevent the draining of your car battery once stopped. The next step is to understand which type of DC-DC charger you require, the size, and installation location. Check as to what kind of power management system you currently have in your caravan. Many of these systems, such as the BMPRO units, installed by the manufacturer, allow for a DC-DC input and if you don’t require high-power charging whilst driving, the trickle charge this provides may be adequate.  This works by energy transferring down to the house battery if the voltage is lower than the car battery. Most likely though, you will need a separate DC-DC charger and not all DC-DC chargers are compatible with other power management systems as many have a minimum voltage they need to detect to commence charging.

The other reason that you will need a DC-DC charger is that by simply connecting to the car battery the house battery back in your caravan will only receive the terminal voltage of what the alternator puts out to the car battery and this will not match the voltage requirements of your deep cycle house battery. This will be a different type of battery – deep cycle, and therefore will need to be charged differently.