You should take your basic essentials including toiletries, toilet roll if not provided, washing up liquid, food, bedding is normally provided but you may want to contact the caravan owner to confirm.
Again researching what is available and ensuring that your caravan suits those travelling with you will make it easier to decide what you need to take. For example you would not necessarily pack a plaster on your holiday if you were travelling alone, however if you have young children then perhaps this would essential to your packing.
If you have children it is worth considering taking a travel cot if required and some toys and things to do for children whilst you are cooking and showering. Board games are great, stories, puzzles, colouring and a few favourite toys. If you have a television in your caravan then check you have the channels that would appeal to your family. You may consider taking a few films or computer games if there is a DVD player in the caravan or computer playing equipment. Always check with the caravan park if you are unsure to avoid unnecessary packing.