Leisure battery Problem

Hi I have a Eldiss typhoon ex300 caravan and i have problems with my leisure battery it is a 1997 model. When we brought it I brought a new battery which worked fine for a weekend away with electric hook up, once I got the caravan home I left it with no mains on in the caravan and the battery went flat I am now on my second battery which also has gone flat any ideas why any help would be nice please thank Paul114