12 volt supply problem 1987 chevrolet r30

I drove my tow rig home from work yesterday, ran fine all day long, parked it and everything died, no radio, lights starter, not even the cigarette lighter. Traced wires all day today, checked all the fusible links, but for some there was no 12 volts getting to the fuse box. Decided to try jumping from the battery to the pair of terminals on the firewall where several apparently normally hot wires come together. Truck cranked and ran normally, disconnected the jumper and the dang thing still works normally even without the jumper now. While I am glad the truck is working, it is not exactly confidence inspiring. I from how it looks (may be mistaken) all the trucks 12 volt power goes through the starter solenoid (both wires from the batt lead to the starter, then some wires go from a second terminal on the Solenoid back to the wiring harness. Could it be an internal fault in the solenoid that by starting the truck jarred it into working am I just completely off base.