I have a 2002 f350 with a 7.3, had some starting issues on it the other day. Charged up the batteries and took it for a test drive, once on the road lost radio, then all power to the dashboard, shortly after the engine shut down. Got it started after a few mins with out issues but once I turned on the lights it shut down again. Today I replaced the alternator on it thinking that was the problem and again after about a min. or so off again. Any suggestions; batteries are good.

 

You say the batteries are good...Have you actually individually LOAD tested them? What voltage are you seeing from them individually, meaning unhooked from each other? What voltage are you seeing from the new alternator?