That's a tough question to answer since the answer is strictly dependent upon to what state of discharge you take your battery(ies).
Observe how deeply you discharge the battery and how many times you take it into a state of deep discharge (<50% state of charge). Further, failing to completely recharge the battery after a deep discharge will shorten longevity.
To further confuse the issue:
Your "state of charge" is expressed in percentage, ie, 0% is empty and 100% is full. Most new, digitally encoded batteries and smart chargers will provide this % on the charger display.
"Depth of discharge" is expressed in Ah. There is a formula for converting Ah to percentage. However, since we generally don't discharge our camper battery(ies) at a constant rate over time, the % formula doesn't do us much good. So I look to "state of charge" on a "smart" controller.
To enhance battery longevity, it is always best practice to never allow your batteries to go below 50% state of charge. Of course you can go below 50%, but doing so will reduce the battery's life span.
