Oricom Battery Sense Monitor
This is the cheapest and one of the most effective ways to keep an eye on your 12-volt system.
It's a Bluetooth battery monitor, and it will let you monitor your batteries from your phone. The Oricom battery Sense monitor, these little units are super easy to install.
All you need to do is connect each wire to your battery, then download the battery sense app on your phone. Scan the QR code on the monitor to connect your app to your Oricom Battery Sense Monitor, it even comes with an adhesive pad on the back so you can mount it securely out of the way.
What I love is no matter where I am at camp, I can pull out my phone and see all the info I need on my battery, like how much charge I have left to run my fridge and camp lights.
I can also see battery voltage cranking health and charge rate, as well as a trip meter to see how your batteries charge while you drive.
I run two of these monitors one on my crank battery and one on my auxiliary battery, but you can connect up to four monitors to the app.
Best of all you can pick one of these up here and then you just have to download the app on your phone and you're good to go.