As said, disconnect the perm 12v+ and use an ignition live from the fuse board instead. Use one of those piggy back fuse holders if you can't find a spare ignition 12v+ to keep things nice and safe. They're peanuts on eBay.
Something like a charger uses about .5w plugged in and not charging.
a 70 amp car battery has about 900w/h so would take about 75 days to drain the battery unless my calculations are wrong. ?