bartamos is correct. Break it into little steps. Check everything. .5 volts at the solenoid will not get it done. You may have a bad batt, a bad connection(loose), a bad ground or a short. You can test the starter off the truck Hey it is only 2 wires and 2 bolts you know. Check the batt under load? Easy turn on the lights and see if the voltage at the batt drops. It should stay at 12 volts with the lights on. So use the list that bartamos gave you and do one thing at a time. And yes that solenoid might not be good. Test that too. Put 12 volts onjust the solenoid. If it jumps the gear out it is fine. If not...you get the idea.
