When preparing the vertical auxiliary panel, I used the opportunity to re-wire the electric box which I had assembled. It had been too small to be closed, so I now used all that empty space in between the panels for laying out the wiring more generously, with better access. I also had bought a new volt/amp meter for the battery which would indicate if current goes into the battery (charging) or goes out. The previously used ampmeter only did show current that did come out of the battery. The new meter is installed in the grey box in the centre. I will still have to decide what to do with the now obsolete previous V/A meter.
I also got another solar controller, rated at 30A, which has a multifunction display and shows more information about the status of the voltages / currents. And I added two green LEDs which indicate the status of the split charge relay: one LED indicates it there is voltage from the front vehicle battery arriving at the split charge relay, and the other LED indicates if the split charge relay is connecting both batteries (when the engine is on and the alternator recharges both batteries).