While the solar charge controller in the van has a volt meter in it, the display is very small and difficult to read from many angles. I've now added a much easier to read battery meter.
The meter itself came from this AliExpress listing. It was AU$5.81 and, as often happens these days, I spent more on the box and connectors than the electronics. It draws very little current and even the backlight turns off after settable time.
Four magnets with holes are bolted to the corners of the box and they work too well - the box comes apart before they let go of the van wall.