There are a number of different types of voltage monitors, which kind you choose is more a personal choice than a necessity. I installed a panel similar to the one at this link. It has the volt meter, a cigarette lighter plug (not heat resistant) and a USB dual plug charger in addition to a power switch. Whichever you choose, make sure you have a way to turn it off (parasitic draw) when not in use. Even though it only uses a small amount of power, when added to all the other draws, every little bit counts.
Here's essentially what I installed, although mine is all in a row rather than stacked two on top of two....
https://www.amazon.com/dp/B01N4QWJUO...detail_3?psc=1