1993 Chevy S10

Battery is at about 13v. Turn on parking lights for 2 seconds and everything goes dark, even after turning off, battery is about 5v. This happens when turning ignition to acc position also. Disconnect battery, let it sit for 15 or 20 minutes and it’s back to 13v. Checked and cleaned grounds, bypassed feed wires under hood, load tested battery (good). Was OK, then did this for a month, couple days ago it worked fine, now it’s bad again. Any ideas?