A digital voltmeter that can capture the max voltage reading makes trouble shooting this problem MUCH easier.

First, check that starter ground is good all the way to negative terminal of battery. If not, repair or replace cable.

Check MIN voltage across battery while cranking. If voltage drops much below 10, battery is either low or defective. Charge and load test battery, and replace if below spec.

When the truck is not starting, capture the max voltages at 1. the input to the relay, 2. the relay output, and 3. each of the two terminals at the starter.

If battery voltage (at least 10 while cranking) is getting all the way to both starter terminals, replace starter.
If voltage is low at the large starter terminal, repair or replace cable.
If voltage is low at the small starter terminal and good at the relay output, repair or replace wire.
If voltage is good at the relay input but low at the output, replace relay.
If voltage is low at the relay input, it is a switch or wire problem.

I cured one Chevy truck that had a loose ground connection, an internally corroded primary battery cable, and a newly installed rebuilt starter that was intermittent (defective). The owner had been getting stalled all over town and was about ready to sell the truck.
When there are multiple problems, a systematic approach saves some screaming and cussing.