Simplest battery monitor and shunt? Advice for non-expert

I have used an Innova 3721 for 9 years now. No installation, no shunt. When you want to know your battery voltage, just plug it into a 12V socket and wait a few minutes for the reading to stabilize. I suppose you could leave it plugged in all the time, but I don't. It can move to any vehicle or camper or device with a live 12V socket. About $20. I write the readings, one for day start and one for day end, into my daily travel logbook.

https://www.amazon.com/INNOVA-3721-Battery-Charging-Monitor/dp/B000EVWDU0