Driving down road truck will die truck will not crank over?

Check the fusible links at the starter motor. They are the soft orange rubbery wires, if there is none at the starter then they are located on passengers side firewall under a cover. Lightly tug on wires and see if they are broken. If all ok there then suspect the connector on drivers side of firewall where the harness goes through the firwall. The bulk head connector sometimes gets green with corrosion. Also you can check the ignition switch itself but usually wiggling it will let it start so I would look for the wire problem

Grab the harness right where it goes through firewall and bend up and down and that should show the problem. The bulkhead connector itself may be covered in thick black under coating but there is a bolt in the middle of it and if you loosen the bolt the connector will come unplugged for visual inspection