Monitoring the battery voltage is great, but if you want to keep an eye on how much current you are drawing from your battery or batteries, a device that monitors current and voltage is better. There are several good monitors available but I have used the Victron BMV700 now for a few years and really like it. There is a "shunt" that you place in series with the negative terminal of your batteries, and the monitor is wired to the shunt.
With this type of monitor, you can see instantly how much current you are drawing from your batteries and turn things off to save battery life when possible. The monitor can also give you a percentage readout to give you an idea of how much time you have before needing to recharge.
If you Google the Victron BMV700 you will find good info.