Assuming your battery cables and posts are clean and tight and the positive cable is clean and tight at the starter. Check the negative cable where it grounds to the engine and run a ground to the frame from the engine.
Next check the power from the battery positive cable terminal to the junction bolt on the right fender. The small wire that goes there contains a fusible link which can burn out and not supply power to any point in the truck. If you temporarily jump this wire and you get power back then you'll need to replace this link. Make sure you do not have a short in the wiring or it will blow again.
There should be another 12 gauge red wire connected with the fusible link and possibly a small black wire if you have the gauge dash and not the idiot light dash. This red wire carries power to a four way junction where the alternator large wire and the regulator red wire and another red wire join. There is another red wire from there to the cab that carries all the power to the cab. It will be seen in the firewall junction block.
From there it goes to the key switch, the fuse panel, the ignition switch, the headlight switch and the horn relay. This will be the key off power circuit. You can check for power at any of those points and tell if the circuit is alright. If the horn blows it means there is power to the cab provided there is nothing wrong with the horn relay circuit.
If all these points check out then I would look at a bad key switch.
Here is a diagram showing what I have just described.
This is the engine bay.
This is the inside of the cab to the horn relay and the fuse panel.
and this is the key switch and the headlight switch. Just follow the red wires.
