I spent a big chunk of the afternoon tinkering in the van - I've installed another voltmeter, so I can check both batteries. I also ran a wire through from the charge relay, so I can see when the batteries are linked, ie one is charged (either by the alternator or by the solar panel). Yay.
Oh, the electrickery graphs:
Power we've paid for
Oh, no DMARC graph, as they've decided to start charging for that. So, um, yeah.