As you probably know, cold weather does some very strange things to normally operating machinery. Voltage loss occurs through a stoppage of charging. So the problem could be in the alternator, a relay/fuse/connection to make it charge the battery, or perhaps it's the battery's time to depart. Sorry I can't be more specific. No insult meant either if you already know these facts.
Good luck. Spring is coming.

'80 4X4 Longbed