You might want to print and laminate a copy of this chart, so that your battery voltage has more meaning.
This chart is courtesy of The 12volt Side of Life (Part 1)
There is a great deal of information here to bring you up to speed. There is also information on this site which will help you calculate your projected daily amp hour usage, which is key to projecting how much usable power you have remaining.
While I have a plug in meter such as Carol suggested and use it from time to time, over the long haul you will want a decent digital Volt-Ohm meter. I would normally suggest a battery hygrometer, but since you are going with AGM, that wouldn't do you much good.
Do what you can up front to reduce usage, such as switching your fixtures to LED lighting
And remember, you don't want to run your battery down below 50%. Sometimes that might be unavoidable, but don't do it often. You will get more life out of your battery.