Fridge confusion!

Hi,  I am new to motor homing and could use some advice on using the fridge.  We have a Dometic 115 which runs on 12v, 230v or gas. I understand that you should only choose 12v when the motor is running to avoid depleting your leisure battery.  What happens when you stop for a short while, say, to go into a shop. Should you switch it to gas as you get out? Is it OK for 30 minutes, 45 minutes? Or will it just stop cooling until you start up again? 

I am assuming that it is always running on the leisure battery but that is Ok when the engine is running as that charges both the motor and leisure batteries.

Thanks for any advice! Robina