It's going to be tough to tell you what that is remotely, but 12.5v while running indicates a problem. Ideally you should see 13-13.7 or so at the battery.
I'd have my battery and or alternator checked...
Course, I've got no idea of what it's like running a truck at -20c


22REturbo.net




1988 4Runner
22RTE core, turbocharged, megasquirted...