I think this is a really common thing. Same thing happened when my van first joined the family... the engine
battery regularly went flat after standing on the drive for a couple of weeks. Being new to the game, we thought it was a fault but it seems it's a feature. We solved it when we got
solar and had a gizmo fitted to divert power to the engine
battery whenever it got low. My first gizmo, years ago, was a '
battery master' but now I've got a
Victron Cyrix
Battery Combiner. These had to be wired into the system to join both engine and leisure batteries to the
solar controller.
There is a simple solution if you still have power to your cigarette socket when the engine is switch off. You can buy a little trickle charge
solar panel that sits in your windscreen and plugs into the lighter socket. They're really cheap and simple. As my van doesn't have any power to the socket when the engine is off, I couldn't go down this route, so can't vouch for efficiency. Hope someone more 'techy' can come up with a solution for you. Good luck and happy camping!
Here's an example...
No more flat batteries. Solar trickle chargers to keep your car (bike, jet-ski, quad-bike) battery topped up. With suction cups for fixing to windows.
www.selectsolar.co.uk