Truck not starting consistently why?

2001 gmc HD 6.6 Have had no issues prior to this, I
have 173 000 on this truck. 92 degree day.
Drove to dump today, turned off, emptied trash,
finished and started it back up then drove to the
gas station. Turned it off for about 15 minutes,
came back out of store, turned it on, started back
up then immediately died. Tried to turn it on again
and it did not turn over. My dash came on, my
gauges worked and it shifted into neutral no issue.
Just wouldnt turn over, wouldnt even click. My only
light that did not come on on the dash was my
WAIT TO START icon for the glow bulbs.  Had to
have it towed home. Let it sit about 40 min, my
husband went out to try it but used the spare key
and started up no issue. Twice.  No delay in turn
over. I went and tried with both 3x each and it
started up also, but took longer to actually start.
Last oil and fuel filter change less than 1000 miles
ago. Batteries 6.months old. I havent started
looking at relays or fuses.
Any help appreciated.