No crank no start?

Hello, have you checked the underside of that fuse relay panel? They tend to corrode a lot being exposed to the elements. A lot of times the wires will eventually just break right off from the pins that the relays and fuses sit in, check that first and if it's clean in that panel we can trace the voltage drop back. Is this a 4wd and what size engine, or just give the VIN and I'll look it up from there to get the correct wiring diagrams for the whole vehicle.
Does the truck have dual batteries as well?

https://www.2carpros.com/articles/how-to-check-wiring

There's no security light at all? Not even when the truck is off and locked up? This is how the fuse panel will come apart, it's very common for corrosion inside these, especially if you live in an area where they use salt on the roads during winter.
Here's your Starter circuit. Diagram 1 is the top, and the other is the lower section.
I've added the OEM diagram just to verify the wire coloring is correct.
I'm sure you have, but just make sure the battery cables are nice and tight, there's a TSB out for it.
Diagrams 6 and 7 are for the Security light and Ignition switch circuit. I think you may find low voltage to other components as well. Refresh this page to see other diagrams as I post them.
The lower dash fuse panel on the driver side is sometimes exposed to moisture or water intrusion if the door or window seals are worn (8th diagram).

Does your Check engine light come on at just the Key On?

Okay, that's good, that means at least the ECM is powering up. If the Check engine light doesn't come on at any point, then you've lost power to the ECM or at least a section of it.

It can be the Park Neutral switch, but test it first, it's a very simple circuit to test, you could actually test the PNP switch and the ECM relay control with one test using the multimeter. The PNP switch gets power from the IGN E Fuse 10-amp while the key is in the Run or Start position. 12volts are going to come from that fuse, go through the PNP switch, through the Starter Relay coil, and down the ECM. But pin 30 is the load side of the relay, not the control side, pin 30 should have power from IGN A fuse 40amp. The wiring diagrams don't show which pins on the relay are the control pins and which are the load pins, do you have a relay that has the little internal diagram on the side? The PNP switch is on the control side of the relay. So, check the ign A fuse for power. I'm not sure why GM doesn't show the pinout for the relays even on the OEM diagrams, but I think you're missing power in a different place.
The relay I'm looking at pin 30 and 87 are the load side feeding power to the Starter, and pins 85 and 86 are the control side of the relay. On the bottom of the relay there should be pin numbers too.
This is a relay I took apart but notice the pin numbers on the bottom. Check to see if you have these.
The 3rd diagram is from a 2005 Silverado, it has the pins labelled. You can see pin 87 is the pin that the ECM grounds to turn the relay on completing the pin 30 and 85 circuit and powering up the starter.

Actually, looking through this other service info, they might not have the pinout correct, but with the key On and the relay out, 2 out of the 4 pins should have 12volts. Does your relay have pinout numbers on the bottom?
Hopefully this is the correct relay you have.
Okay, finally this grey relay is an OEM Chevy Relay, pin 30 is the load, it should have power coming from the IGN A 40amp Fuse.
Sorry for the confusion there. There were a couple errors in some service information.

Disregard that 3rd wiring diagram, which looks incorrect, I'm not sure who labeled that, but it could cause a serious issue if someone tried jumping out the relay pins. I can't seem to delete it.

Check for 12volts at PNP switch before installing a new one, it will have 12-volts on the pink and dark green wire with the key "on" if it's working.

Technically speaking, with the starter relay out, and key "on" you should have 12volts on 2 of the pins where the relay sits. Thats the fastest way to check all power inputs to the relay.