Yup old timers could tell that the 6volt batteries would die slowly. You could turn the starter until it would quit. Wait 5 minutes and turn it over again. This newer 12 volts die quick, and since your alternator was still charging it just wasn't taking a new charge.
Check your voltage on the alternator to be sure it's OK. If it is too high your diodes could be bad, you will get AC current which would not charge your battery. A professional charging test equipment can check that allso. 
2002 GMC 1/2 ton Sierra,4WD, Xct cab, camed, mid-length headers, Nelson tune, electric rad. fan, trany cooler, Heavy 5750 lbs with topper.