I would go on google earth, you can check past images up to ten years, use any facebook images etc to prove how long the household items have been there, and go for retrospective planning permission for amenity use. It would make your land more valuable. If you can prove the caravan is not movable, you can say its permanent.
My neighbour has been living on his land for over four years and that is what he has done, they had to give it to him.