Not familiar with the 'dual on a half ton' setup but I don't see why you couldn't run a 3rd battery using an isolater so it would charge with the other batteries but not discharge the others if you run a load on it.
Thus far the stock batteries have been holding up well to a 2500 watt inverter which is only used to run a coffee maker and a microwave while camping; I'm careful not to run them down to the point where I can't start the engine or do permanent damage to them. I have an Optima yellow that I charge off of the aux power on the trailer hook up through a charge controller for running other stuff in camp.
A forum community dedicated to Dodge Cummins Diesel truck owners and enthusiasts. Come join the discussion about reviews, drivetrain swaps, turbos, modifications, classifieds, builds, troubleshooting, maintenance, and more!