If you have invested the mod-time and mod-money to upgrade your battery bank and charging system, then you should also invest in a battery monitor. There are a variety of monitors available covering a broad range of features. A simple volt meter can quickly tell you at a glance what your battery voltage is and there before the assumed percent of charge remaining. This can be a bit misleading because since voltage alone does not tell the whole story. A simple voltage reading would need to be performed when there is no load on the battery for at least 24 hours to get an accurate reading. This is not very practical and probably not something you would want to do repeatedly.
What is needed is a way to tell how much capacity you have left in your battery bank, much like a car's gas gauge. At a quick glance, you can see how much gas you have left so you know when to fill up. Battery monitors work much the same way. Not only can they tell how much battery capacity you have left, but they also can monitor real-time current draw, how much power has been consumed, and how much current going in to the battery during charging.
Performing this mod requires both patience and high quality electrical connections. These type of battery monitors use what's called a shunt, which is a high-precision resistor that goes in between the negative battery cable from the battery bank and the battery bank terminal, such that all the current going into the battery bank (charging) and out (discharging) must pass through the shunt. The current difference between the shunt terminals is measured and sent to the monitor for display. It is essential that all connections are properly made to ensure accurate readings. It should be noted that the shunt, wiring, connectors, and mounting boxes are all sold separately so plan on purchasing these items in addition to the monitor itself.
Mounting options for most monitors include box mount and surface mount. Surface mounting provides a clean and finished look. Just be sure you have enough space behind the mounting surface to accommodate the monitor circuitry and connections. Follow the instructions provided with the monitor as they usually come with many diagrams and examples, especially the TriMetric brand.
After you have installed the monitor, the fun begins. There is a bit of tweaking to be done, like programming in your battery bank capacity in amp-hours, charging parameters, time-to-discharge sample intervals, and alarms. After a few discharge-charge cycles, you may find that you need to tweak the parameters again to get them just right. But modding your RV is supposed to fun, right?