Hello everybody, chevy express 2500 4.8 don't want to start some times it does

Thanks for replying, when the van is running I check the alternator and it's sending 13+ volts I checked the battery and it's getting power from the alternator but sometimes it just sounds like the battery is dead, change the starter and it does the same thing I'm starting to think it may be the ignition switch but not sure, or maybe a ground wire not making good contact any suggestions what could be the problem. Thank you

Should be 13.9 to 14.5. Maintain under load. 13.0 is not enough.