Originally Posted by lswartz
Just go & see what happens. When your run out of power, go home.
I understand what you're saying, but not everyone will. I knew a guy that just ran his batteries down until the lights started to dim and that's when he called it done. He replaced his batteries once a year and thought that was normal.
I would suggest, if going this route, to take a multimeter with you and check the voltage once a day to get a feel for it. Check it when there hasn't been a load on it for a few hours, maybe after a little trek away from the camper, and when it hits....say 12.25V call it done. It's not the most accurate way to check, but probably the easiest way to get a feel for it. Better idea yet, is to try that at the house and see just how much of a hit they take before you actually try it for real.