need some help

[fordpower21]

Hello,
I’ve been having a problem with my truck for the pass month now. It’s really annoying because I can’t pin point the problem. I have a 2001 f250 with a 7.3 motor. The problem starts out when I’m driving my truck for about 40 minutes, and then my battery gages start to drop, making the rest of the other electoral gages drop as well ( fuel, speed, lights, radio). However, if I have two fully charged batteries, I can manage to drive for about two day without any problems. During the second day, my problems start to occur. Cables are all in good shape. I have two new batteries. I changed out my alternator about a month ago when the problem started too happened. I bought the alternator from kragen.
Last night a buddy of mine came over and told me that the alternator wasn’t pushing out the right voltage. Then this morning I took the alternator back to kragen. The workers tested the alternator and it show that everything was fine. Can a good alternator still not produce the right voltage? I’ve heard stories on the form that some alternators won’t work properly because there not Mortorcraft. Is that true? What should be my next step? I really have no idea. Plus, I just spent a **** load of money on things that I thought might be the problem.
Thanks,
Fordpower21