Motor Home Electrics

I am new to Motor Home owning and I am having difficulty getting my head around the 12 volt. 240 volt use? I The way I understand it is you get 12v DC off the leisure battery and 240 volts AC off the mains. So when you hook up do the sockets on the Van become 240v and if no hook up they are 12v. Can i use a 12 volt appliance when hooked up or can I only use 240v and vice versa can i only use a 240v appliance when hooked up. Do I need to buy a 12v kettle toaster iron etc if i am going to use the Van 'wild camping'.As you can see i am confused.