If you are constantly connected to electric hook up and running just the lights, water pump, toilet flush and radio then then a 75 ah battery will be quite adequate. You only need a bigger amp battery if you intend using a motor mover as they consume around 80 amps when moving the caravan over a short distance.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.