Car turned on and ran fine but once off battery dead? Ford 390?

I have a project car i'm trying to figure out,
It turned on today no problem and i drove it for about 20 minutes and when i got it back home i turned it off, i went to turn it back on and battery was completely dead, multimeter read below 12 volts i think it said 11.94,
Battery is a bosch from may 2015
I put in a new alternator and voltage regulator both are grounded well
Battery connections are good and tight

The only loose connection i found was from the engine block to the body of the car would that cause a non charging issue?
I'm trickle charging the battery now

Get a new battery and test the charging system.

You also have a Battery to Engine Ground, right? Yes, any bad ground will cause problems. Check the car running…14 Volts is what you want.