How old is the battery?
Do you charge it with a "good smart" charger two or three times a year, as recommended by most caravan battery suppliers?
How long do you leave it, before it goes flat?
What is still connected? Perhaps your Alarm system? A battery, fully charged and in good condition, should maintain the small current taken by an alarm for a couple of months.
Have you tried disconnecting the battery completely?
Even a disconnected battery will self discharge, depending on age/state. A battery in v good condition will be OK for a few weeks/months after it has been fully charged.
Cheers
Dave Mac