From one 12v battery nerd to another 12v battery nerd: :) I have been playing with my batteries for quit a few years and here is my situation; I have 2 6-volt batteries that are rated at 225ah. When fully charged, rested and tested, they read 6.37v. In reading from the 12 volt side of life, I learned that a volt reading determined a percentage of state of charge. Now here is an assumption that I know is not correct, but I will use a straight across association of percentage of state of charge with amps used. So for me a 50-percent state of charge means that I have used 225/2 = 112a.
So I tested all my 12 volt things for amp usage to get a general understanding of what uses a lot and what doesn't.
We went dry camping in conserve mode with LED lights and most all of our time was spent out side. Took 2 navy showers per day and it takes 6 days to get our batteries down to 50-percent.
So now, after all that testing and assumptions, I will now check my batteries with my simple volt meter once every couple of days to see if the numbers still make sense. And if so, I go back to enjoying my 6 or 7 days of dry camping.
