We had a full cover a few years ago which as tailor-made to size, so therefore not cheap. The second year we used it it marked the caravan all over, with blotches which never really polished out. I would therefore consider a roof only cover had I your trees to contend with. Protec do one which again is not cheap and which is secured by sliding into the awning rail on both sides. There is one where we store our van and it looks quite good. There is also one on the market with a slightly fuller cover which seems to have elasticated sides, but again they are not cheap. If the trees are a real problem, then a cover might well be worth it.