Contrary to some advice, with a correctly designed "smart" charger leaving it connected continuously is the best treatment you can give your lead acid battery.
Some chargers/controllers fitted to vans are anything but correctly designed. Certainly those on older vans had no multi-phase charging or load side voltage regulation. These used to sit at 13.8 volts boiling the battery and should not be left connected after they part charged the battery.
So whether leaving it connected or not solely depends on what kit is fitted.
Returning to the OP's question: because other batteries are flatted whether the original battery is wreaked or not is a side issue. The problem is something is drawing too much current, far too much.
The cheap device shown in the following link is a bit of kit we should all carry in our tool bag. It enables you to systematically measure what current each fuse is carrying and with this and a modicum of thought you will quickly identify what item has gone wrong within you 12 volt system and is burning up your stored power. With this device you remove the fuse being investigated and fit it in the device, it then functions as normal and the current flowing can be read by pressing the button.
http://www.maplin.co.uk/Module.aspx?ModuleNo=217879