Power problems

My fiance just put in a new alternator a few days ago and it ran fine until I drove it the next morning to take my son to school. Everything seemed okay until I approached his school where the car started losing power. Everything started quitting one at a time beginning with the radio so I turned everything off except the engine. As I was trying to get home it started raining so I tried the wipers but they would not work. Every time I stopped the power would drain even more until finally losing all power right in the middle of the road where I could not do anything at all, not even the emergency lights. We have already replaced the main bus fuse and the battery but it still will not start. Any suggestions?