switching between 12v & 240v

Hi folks, doing up a van to live in temporarily, I bought a Waeco cr1140 f/f. And as a noob hoping someone may have an answer, as the manual didn't specify. :/
Question is: Do I need to switch off 12v when connecting 240v....can I leave them both on or do i need to use a switch between them both ?

Also is it fine to leave it permanently connected to 12v if battery supply is not an issue , or run 12v batterys through an inverter to produce 240v ?

Any help would be appreciated immensely
Cheers