Will not start, truck will click once when trying to start?

This sounds like you have a defective starter, to confirm the issue use a test light on both the main power feed and the trigger wires if they both maintain close to 12 volts the starter is bad, here is a guide to help you I have included the starter wiring diagrams below. Also, there is a starter relay I would swap out to see what happens.

https://www.2carpros.com/articles/starter-not-working-repair

Check out the images (below). Please let us know what happens.

I replaced the starter with a new starter, not remanufactured, swapped the relay and I still have just the one click at the starter. I am stumped by this. I have battery voltage at the starter and 10.5v from the signal wire when attempting to start the truck. Any other things I may need to check. I can manually rotate the engine. Everything in the truck lights up. New ignition switch. Anything else I can check.

You should have 12 volts at the trigger wire. As you can see from the wiring diagram. I would use a jumper wire to trigger the starter solenoid to confirm the starter and main batter cables are not the problem. please let me know what happens.

When you mention trigger wire are you referring to the wire from the ignition switch? I have 12v from the signal wire from the ignition switch wire, then when someone tries to start the truck, it drops to 10.5v. I have 12v from the battery to the starter. I will attempt the jumper wire and test resistance in both the positive and ground cables.

The trigger wire to the starter solenoid. Run a jumper to see if the engine cranks over, please.