I assume you mean for running your inverter and electric heat or A/C. They will weaken over time and you will see the length of time they hold enough AMPs lessen to the point you need to restart truck to charge after about 24-36 months of use. Again that depends on how hard they are used.
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About how long do the batteries last in these trucks? By last I mean how long before they need to be replaced because they wont hold a charge overnight. I know some do better than others but just a rough average is good.