I plan to ignore the invertor and use the charger for the battery. Then take the 12vdc to a fuse, and a chassis socket.
An external box will have:
A: socket for 12vdc input and a switch to a voltage meter
B: 5vdc Buck converter for Raspberry Pi, or Arduino to a switched socket
B: 5Vdc Buck converter for powered usb hub for keyboad, mouse, external usb harddrive, ect. to a switched socket
c: usb charging module, also with its own switched socket.
Note: A: and B: could also be for powering breadboard circuits
An external box will have:
A: socket for 12vdc input and a switch to a voltage meter
B: 5vdc Buck converter for Raspberry Pi, or Arduino to a switched socket
B: 5Vdc Buck converter for powered usb hub for keyboad, mouse, external usb harddrive, ect. to a switched socket
c: usb charging module, also with its own switched socket.
Note: A: and B: could also be for powering breadboard circuits