Truck shut off going down highway and it has spark air and fuel and the engine spins nice and quiet as if there is no combustion or attempt to start. It has always ran smooth as a button and timing chain and heads were done 50,000 miles ago on this 5.3 ... any cause that would keep it from starting

Sounds like a broken timing chain. Valves are open so no compression, which is why your truck cranks so easy and quietly. The knock sensors may have cut the engine off when the timing went. Try a compression test and see if those valves are closing/opening.

I also have compression and timing is right as it appears what else can update be issue...

Did you ever figure out what was wrong? My truck is doing the same thing.

try the crank sensor,if its bad you get the same results

If you do not have a compression tester pull one of the valve covers. Make sure all are tight and snug while working. You will need 2 people to do this