No electrical power after replacing the alternator and fuse?

Was the truck running for 20 minutes without it connected to the jumper after it started? If it did, I would check the battery terminals and wires, sometime a little corrosion that you can't see in one of the cables. It may be a loose or corroded wire that is not making a connection. Try grounding the negative side of the battery with a wire and then to the ground side of the frame.

Yes, it was then completely shut off. Also, I can't seem to get the positive side of the battery to light with a test light. Could it also be a bad positive terminal?

Disconnect the battery on both terminals, then try the test light to see if it lights. The batteries you had tried were new or used and had been charged? If you do get the test light to go on clean the terminals going to the battery.

Both were new.

Also, one thing I am not understanding is the positive wire actually has a fuse that I replaced yesterday, but when I connect that terminal and use a test like the fuse isn't even hot.

Was looking through a couple of GM, try taking the wire with the fuse and ground it out. If it is hot, you should get a spark from the wire where you ground it out.

Okay, so, I actually have fire at that fuse and wire, but the light on the test light was very dim, so I'm thinking maybe I have a break somewhere in that wire is why I'm not getting any power to the vehicle at all. Does it have a main fuse or fuses I can check?

There is a fusible link connected to the battery. Check it for a bad fuse connection.