1992 Nissan 4X4 Pickup 153K miles

Hi, I recently purchased a 1992 nissan pickup from a friend with 153K miles on it. He put a new battery in the truck because it had been sitting a year, some gas, and it fired pretty much right away. He drove it 30 miles to my house, and then 30 miles home, without problems. Then he drove it 20 miles to a neutral place where I was able to pick it up after purchase. I live in Colorado and it has been in the low single digits of late. Anyway when I picked up the truck it was slow to crank, but it turned over. I drove it around town a bit, then drove it home. I went out that night (first time I turned on the lights) I left the lights on by accident for 5 minutes while doing something. When I went to start truck, dead battery. So I got a jump and drove 5 minutes home, by the time I got home it was dead, and I had to jump it in my driveway to get it into the Garage. I took out the alternator, had it tested at NAPA, they said it was firing perfectly. It was a little loose and was missing a nut which I fixed when I put it back together, the belt seems to be working fine and turning the alternator. SO the question is, if the battery is fine and the alternator is fine, what is my electrical problem? Any ideas would be appreciated, or any help. Do you even think it is an electrical problem, or do I need to just get a good charge on the batter to give the alternator a chance to work?