A more detailed walkthrough will be coming soon, but the basics that need to be checked.

  1. Using a test lamp clipped to battery +, Probe pin10 in Connector B (generator field driver), Look for a GND control signal when charging should be active. You can use scan tools like AUTEL to actuate the control signal if necessary.
    IF YES, Its good. Move to step 2.
    IF NO, PCM needs repair. 

  2. Using a test lamp clipped to battery -, Probe pin25 in Connector C (generator Output from PCM) for 12V. 
    IF YES, Its good. Probably a loose connection or bad alternator. You should seek out more mechanic oriented resources. 
    IF NO, That's bad, You will need to investigate the ASD relay. This is what supplies the 12V output to pin25 on connector C. 

    NO ASD relay supply also causes no tachometer and no switching of the transmission relay, along with some other problems.

    I will add more details on checking the ASD relay when I get a chance but for now I will leave you with the schematics. Keep in mind, for these trucks the pcm needs to see a crank signal from the CKP to actuate the ASD control signal from the PCM.  So for basic testing check relay control signals and supplys to see where the problem is and go from there.