[madmike]

Meditating on this post and missed the obvious. Your Alternator was fluctuating. When running , your alternator should be charging the battery as well as running all your electronics. New batteries? No drain? really sounds like a bad alt. Im sure any local auto parts will check it for you. Although I usually place a voltmeter across the battery to see if its charging. I believe it should read above 13 volts constant. Seems your truck is running off battery until it dies.