I don't know a lot, but I think for others you better clarify a fe things, like is it just parked there or being lived in, how gar away are the neighbours, etc
Its a temporary structure, so no regulation - unless there is something in the property land title as a restriction preventing the keeping of caravans on the land (many housing developments in the last 30 years have such a restriction).
Unless someone is living in it - then it becomes a planning issue and the local council will be interested.