1999 Ford F-250 battery issues

I have a 99 Ford F250 7.3 Diesel. I have replaced both batteries two times in the last week. The first time I replaced them I would go uphile slightly and not at too high of RPM's and the battery light would flicker. I had them tested they were completelty dead replaced them again and same thing! Uphile light would flicker and it seems to loose a bit of power. Alternator is still in great condition? Help!