There are contributors more expert than I in batteries, but the most important thing is that the caravan battery is NOT the same animal as a car battery. Caravan batteries are usually know as leisure batteries.
once this is established, it depends on your pattern of use of the caravan and whether you have a motor mover fitted. The generally accepted minimum rating for such a van is 80AH (ampere-hours) and many of us use 110AH. The latter is physically bigger than the former so check sizes and dimensions of your battery box.
Ideally leisure batteries should be kept on charge with either the charger unit in the caravan or another specialised charger but NOT a standard car battery charger.
If you are a Caravan Club member, go to their website and download their technical leaflet on batteries - it explains things far better than I can.