1987 Ford F-150 Two dead batteries?

1987 Ford F-150 280,000 miles
for about a month my car's battery meter has been in the red zone, at 0v. Recently, I had to replace the battery because one morning the truck wouldnt even crank. I put a brand new optima yellow top in, and today it died, so I am in the process of recharging it. I dont know what could cause so much voltage drain on such a seemingly "tough" battery to wear out. I didnt do anything out of the ordinary when I was driving it. When I turn the key now, nothing happens. The battery is shows 8.4v and the engine cranks twice slowly then cuts off. When it does start it misfires and after I shut it off I cant start it back up for at least an hour. I have a feeling that all these problems are related to one bad part but I cant figure out what part it is.