You'll need a DC voltmeter. Check batteries first wqith key off... should be over 12.5V. When you turn key on, it may drop to 11.3 or so until glow plugs turn off, then within 15 seconds return to low 12s (12.3, 12.4) Now start it (if it will cooperate) and voltage should soon level off in 14.4/5 range. If so ,everything's good, if not, time to troubleshoot.
Look for bad connections at battery and alternator. Check for belt slip. Do you have volt gauge on instrument panel? Don't know if our charging systems are set up this way but, if charge indicator light is burned out, it could prevent alternator from "activating". If you only have the warning light, make certain it comes on for few seconds when you turn key.
After you get this resolved, please don't start your truck everyday just to hear it run.
It's hard on starter and just wasting fuel and probably uses more battery than what gets restored. Just leave it on trickle charger till you need it.
Just my oh, so humble opinion... lol
Let us know your progress. Lots of experienced folks in here.