Hello Chris,
Most caravans do have a power supply fitted (battery Charger) which should chareg the battery when mains is supplied. It is necessary to ensure all the relevant power switches are set corectly, and for that you should refer to your owners manual.
There could of course be something wrong, a fuse may have blown, or if the battery has been discharged or over charged for some time, it may be permamantly damaged and need replaceing.
As a general rule most chargers will have some sort of 'Mains power on' indicator. If this is active then mains is ok, and it points to a problem on the 12V dc side.
If you are not confident with working on the elctrics I strongly suggest you use the services of an electrician who should be able to identify the fault and recommend the best course to repair it.