82 diesel Camper

I want to use the 2nd battery compartment under driver, now empty,
to charge and run digital gadgets, phone and laptop...
~battery that charges while driving
~inverter for usb and 0.5amp 115v recpicles.

Has anyone done this nicely with photos or diagrams or parts list?

I still can't find the 2nd battery relay detailed in Bentley. It is in my
empty driver's seat base hooked up to the refrigerator fuses.

Frank Lee