I've got a 12 volt "cigarette lighter" LED voltmeter that I used to use in the power outlet on our TV antenna booster. It worked well, I could simply "pull it out a little" to disconnect it when not in use (saves battery power that way) or push it into the outlet, read the battery voltage and know the battery charge condition much better than the "system status panel lights"...
I got it from Amazon for about $5
https://www.amazon.com/Universal-Bat...led+volt+meter
Since then, I've installed a "power panel" with a built in voltmeter, USB charge ports and an extra power outlet.
https://www.amazon.com/Cllena-Functi...led+volt+meter It's a permanent installation with a off/on switch to save power. For us, it works better than the "cigarette lighter LED voltmeter" but I still have that one stashed away in the trailer.