battery totally flat after installing solar panel

Hello. We bought our caravan 3 years ago and we left the battery in the caravan over winter and it was still virtually fully charged the following spring. We then had a 80wp solar panel fitted and the following spring the battery was fully flat and the dealer replaced the battery at no charge saying the battery was duff. Just been over to the caravan storage and the battery is again fully flat. Its a Bailey Pursuit 400-2 and the solar panel is a PV Logic 80wp. We do have a motor mover but never use it to save the battery. The van is stored at YC Leisure and they service it but try to blame us for not taking the battery home (we only bought the solar panel so that we did not have to take the battery home)

Please have you any advice