are you ensuring someone is activating the ignition switch all the way? Not just key half on on ACC. Also make sure your meter ground is grounded good. If the wire is not getting the volts when key is turned all the way on, then try the same thing close to your ignition switch, but between the ignition switch and starter solenoid. Just stab the positive probe in the wire insulation and ground the other well.
that will narrow down if it is the wire or ignition switch.
You really should be getting the volts to the solenoid though. Do you keep extra starters and stuff for your truck? I have 2 starters, 2 alternators, 2 distributors, everything...thanks to a nearby salvage yard and cheap parts. The first thing i would've done is spend 15 minutes and replace the starter and solenoid with a spare and see what happens .