To me that implies the problem is at or very close to the battery.
Hopefully you own, and know how to use, a Volt-Ohm Meter (VOM).
First step is to compare voltage across the battery terminal posts with the voltage across the battery cables at the battery. They should be the same value. You are trying to make sure the batteries cables are properly coupled to the battery posts. Many times the problem turns out to be bad connectors.
Once that first step is done troubleshooting will have to be somewhat methodical. Start at a good known end, then a bad known end. Find someplace in between and if still no good, move closer to the good end until you find the break.
I don't have a wiring diagram to match your truck, so my help can only be somewhat generic.
Switch to the Ohm scale and check between negative post on battery to some other metal (unpainted) surface such as the engine block and/or a bolt coming through the firewall. If you can, check where the cable attaches to the body. That will verify if the ground side is working. If not you will have to check ground cable continuity along it's length.
If ground is OK, then you have to move to the positive side. I would suggest looking in the under-hood junction block of fuses and relays. You want to use one that is not triggered by the ignition switch being active (brakes, emergency flasher, etcetera). Remove the fuse and test to see if there is continuity between the positive battery terminal and the hot side of the fuse block.
Not being there and not having a wiring diagram limits my thoughts on this. You will have to find where the wiring has a discontinuity and fix that. Go back to what I said about a methodical approach.
