How do know when your battery is on the way out? I check it for water and check the light meter to make sure its charging when the trailer is plugged in .
Get yourself a battery load tester. $30-$40 at any auto place or Amazon. Handy gadget to have around. It has a coil in it like a heater which puts a large load on the battery. You look to see how fast/if the battery recovers voltage after the load is taken off it.