Firstly welcome to the forum, we may not be a busy forum but we're a friendly lot...
12 V lantern or do you mean a rechargerable lantern ??
We have a Primus 9 watt rechargerable lantern, which can be recharged via 240 V or 12 V, I have a timer set in my shed on one of the power points that only runs 15min's a day I leave the 240 V charger on that and just plug the lantern in when we come home from camping, it tops it up and doesn't over charge it.
While away will have a solar/battery/fridge setup and if the lantern loses power I plug it into the battery or if we're driving around I plug it into the car ciggy lighter and it recharges in approx 2 hours.
The light runs for approx 15 hours before needing to be recharged, we have never flatten our lantern yet.
The only thing I wish was around when we bought our lantern, is the remote switch one's, as our switch is on top of the lantern and we hang it up hight to light under our annexe and is a pain have to lift it down and up all the time to swich on and off.