My truck won't start, not even a click. Could it be a relay? on 1998 Dodge Ram 2500

It's got a 24V Cummins diesel engine. The truck was running earlier in the day, though it was hard to start, that is it took longer for it to turnover. The batteries look to be good and no obvious burnt fuses. The glow light comes on and then goes off, but when I try to start it, nothing. It made an odd kind of low growling noise the last time before it finally did start, but now there's nothing.

Asked by for the 1998 Dodge Ram 2500
Check voltage at starter motor, remove batt. cables, load test each battery to see if it is ok,
if ok clean and tighten cables. If voltage at starter ok "TAP"starter with hammer while
someone tries to start eng. if starter tries to turn REPLACE starter motor.
Thanks, did the first part, batteries and cables OK. Haven't a clue as to where the starter is but my husband remembered a good friend who is a mechanic and he said he'd check it out for us. Thanks for the information, it is greatly appreciated.
You are welcom.