have an f350 6.0l. After oil change, truck will randomly not start, (no 12 volts to the starter). I have looked at a schmatic or the starting system and with the elimination of the starter, the park/neutral switch, starter relay and ignition switch are the only variables left. Does nayone know where the starter relay is located?

The starter relay is the same thing as the starter solenoid, which is located on the starter motor. This link may help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235560

Diesels require strong batteries. Remove and fully charge both batteries. Most auto parts stores will test and charge batteries for free.