1993 GMC C3500 SLOW CRANK, HARD TO START

I drive a 93 GMC 3500 pickup with a food delivery box on it. VIN #1GDKC34F1PJ516187 with a 6.5 diesel turbo. In the past week it has gotten where it is slow to crank and hard to start, first thing in the morning, during the day on the route or when I take it back to the shop in the evening. It initially gave the indication of battery or charging system problems so I load tested both batteries and they tested fine. I then load tested the charging system and all voltage and amperage readings were good. I load tested the starter and it showed good as well. In the past week the "glow plug" light has only intermittently been coming on, not really very much at all. I'm not that familiar with the diesel glow plug system and controls and need some advice regarding how to diagnose the problem.