You have to try to enter all 24 campers/trailers/caravans in the Holiday Camp. The doors themselves are a bit bugged (not always giving a sound when you try to open them), your clue will be the lock sound - if you hear that, you' can move on. Some of the campers are accessible and you can even find 2 radios, a TV and a motion event in them.
It's recommended to do this while going for the Completionist achievement.