I will assume that you have enough battery power for your needs and that the batteries are in good shape. There are several great devices to measure power consumption. Xantrex makes a couple of popular models. 

I have an older Link 2000 on our boat — it will monitor two battery banks — and I use it often. I check current use whenever I disconnect from shore power. I can see how much charge is actually going into the batteries from alternators while I am running or using my generator. And, most importantly, I watch power usage when I am running solely on battery power.

It tracks several aspects of battery usage. When the engines are shut down at the dock or at anchor, I turn off all of the non-essential power devices, including the inverter except when it’s needed. Then I check how many amps are being drawn from the batteries. Inevitably, I find something that is drawing power that I can do without and turn it off. 

The Link also keeps track of total usage and shows voltage. You can program how many amp hours you have available, and then it will tell you how much of that total is used and how many hours you have left at any particular draw. When you look at voltage and current amp draw, it is a great indicator of your battery’s condition. As an example, if you have a battery bank that can supply 150 amp hours before requiring a charge, it is very telling what voltage exists once you get close to that 150-amp-hour mark. If the voltage is not staying above 12v, that can tell you your batteries are no longer working to their capacity.