Odd issue.. could use input

[Mgicm]

So couldnt find the exact forum to post but think it is more of a general issue. GMC 1973 truck... I went out this am, started without issue and drove 14 miles to our city. Shut truck off and came out about 10 mins later. Would not crank. Dash lights ok. I thought for some reason it was a hot start issue where the starter needed time to cool down. Why? A few other times this happened, we let it sit for 1-2 hours and it cranked right up. Today..three hours and nothing changed.

So finally had someone come out with a meter and jumper cables. Battery (2 months old) showed 12.5 volts. I gather not enough to start this chevy 350? Jumped it off friends car and she started like nothing ever was wrong.

Drove home 14 highway miles...still showed 12.5v. Would that immediately say alternator problem? Nothing was on such as headlights, radio, etc. Nothing. Alternator was just replaced 2 months ago. Or does this possibly relate to a battery issue like not holding a full charge...or something draining it ? I can check voltage tomorrow am.

Sorry for basic questions..

thanks GP