With lead acid batteries, you cannot use the full capacity of the battery. In practice, you can typically only use about 50%-60% before the battery voltage drops too low to be useful.
In addition, even if you only use 50% of the rated capacity, you will kill the batteries fairly rapidly. If you limit yourself to 30% you will get a much longer lifetime out of them.
Using quality deep cycle batteries (think fork lift batteries) will help with as far as how far down you can discharge them, but they still do not like being deeply discharged and will die sooner and degrade faster the more deeply you discharge them.
If you get quality deep cycle batteries, I would not count on more than 50% discharge and 30% will be much better. That means you need more batteries to do what you want.