You did step one= have the battery tested.
If the truck was sitting and not started and you started it by either hooking a charger or jumper cables to it you first have to check and see if the battery is actually fully charged or drained when you go to start it when it is cold.
First I would clean all of the battery cable connections and have clean bare shiny metal under every connection. That means SCRAPE the spots where the ground cables contact the block or frame to CLEAN BARE SHINY METAL. I've gone out and checked three or four just got it together and now it doesn't want to crank good vehicles and every time the issue was because the paint was insulating the ground strap from the block. I actually had one guy cry and fuss when I pulled out my knife and scraped the paint off from under the ground cable and put the cable back on. The car started right up though.
Then get out your multimeter and fire the truck up and check the voltage at the battery, Remember generators don't charge much at an idle and you need to speed the engine up a bit to see if it is charging.
Click on section 12
http://chevy.oldcarmanualproject.com...ruck/index.htm Then you can either read page by page though how the charging system works or go to page 12-8 where the trouble shooting and testing part starts and continues for several pages. I'd read start to finish just to learn how the system works. They wrote the old manuals in very basic language and the good thing is that they didn't assume that the mechanic working on the trucks had any experience. It is almost as basic as the how to repair your VW for idiots book that came out in the 70's.
The contact points in the regulator can arc and stick just like ignition points. You can clean the contacts up with a point file or snag a small emery board and use it.
If the generator hasn't been freshly rebuilt the brushes may be worn down and need replacement. They aren't hard to freshen up but can be a bit of a challenge the first time or two a guy does one. The fun being figuring out how to get the brushes to slip back over the commutator of the armature.