I think he should start with the basics, and use a volt meter at the back of the Alternator to see if it is putting out the right amount of volts. Do it at Idle, then do it at 2K RPM with everything on, headlights, high beams A/C EVERYTHING and the voltage shouldn't drop below the number that was recorded at idle, but it should be higher. Then dissconnect each battery and test them with a volt meter.
Just because an Alternator is 6mo. old, doesn't mean it's good! I've seen plenty of brand new junk parts that are bad, and they are usually alternators or starters.
And the Optima batteries are not good at all for these rigs. I just replaced both of mine (and a new alternator) and I went with the 850 cc amps from Motorcraft (ford replacement batteries, and alternator)
If everything checks out okay I would point to the stereo system first, if there is an aftermarket amp installed, some brands are notorious for not shutting down and drawing power all night.