With key on, 5 to 8 volts at inertia switch is NORMAL, its the monitor voltage from the computer
Did you do 50/50 test?
Spray fuel into the intake and then try to start engine
If its starts and then dies Spark/compression is OK and no fuel is the problem
If it does NOT start then its a Spark OR Compression issue
50/50 instant results, no guessing
Do this test FIRST anytime you have a crank but no start issue on any engine, lawn mower or Ferrari, cut your diagnoses time in half
When you turn on the key you can HEAR the fuel pump run for 2 seconds, but just for 2 seconds, its not quiet, a HUMMM from back of cab area
Repeat key OFF and ON as much as you like to either confirm you hear it or don't hear it
If you have a volt meter test the battery
12.8volt is a new battery
12.5v is a 3 year old battery
12.3v is a 5/6 year old battery and time to shop for battery sales
12.2v or lower is a dead or dying battery
When a starter motor is activated battery voltage will drop 2.5volts because of the large AMP draw of turning the engine
If system voltage drops below 9.5volts during cranking some electronics will reboot and cause a no start, from low voltage, as said by Dirtman