Permanent camping – Termination – Eliminate foliage

Hello, maybe someone here knows about the rights and duties during the permanent camping. My grandma and my grandpa (both almost 80 years old) have now (so this week) quit at the campsite. Tomorrow the caravan belongs to someone else who places it somewhere else. The old place is then empty until April. By April he still belongs to my grandparents. The placekeeper now demands that my grandparents free the place in April from the foliage, which I find grossly outrageous. After all he gets for nothing money in the throat and my grandparents are really not the youngest anymore. (At someone else he will hardly rent it in the time, because the place is not particularly well visited) Probably this is their duty, because the place legally still belongs to them. Can one nevertheless somehow get around it? Many greetings, Nadine