My Battery is 12 months old, and often shifts 800l of water. and it is and 85ah battery too.
Are you sure all of your connections are rock solid? Weak connections will really affect your battery's performance.
I have no idea about your charger, but with Leisure batteries it is always recommended that you use an intelligent charger.
I put mine on charge every night, and it is charged up by the morning, now and again I have forgotten to charge it, it starts to fade about half way through the day.
I just connect it up to my van battery with jump leads.
I'm fortunate in that the position of the van battery is close enough that I can rig it up to my leisure battery.
Go to a caravan supplier and get your battery checked out, maybe it is knackered, maybe it isn't.
Ian