Stop in for a cup of coffee

My cousin is a carpenter and have a van, and with the doors open for a long time when he is in and out the battery goes flat. So, he asked me, what would be the best way of solving that. There is no switch in the car to turn off the lights. He asked if he could have an extra battery with some kind of relay so the relay will switch off when the voltage is low enough and thereby cut off the extra battery for the lights from the battery the car need to start with. The problem with that is that when the voltage is that low, the car will not start anymore either.

I wonder if a timer would be an idea, open the door or push a button, and the light is on for so long and then goes off.

Bill