I'd say your alternator is done for. They'll usually go when it's raining and they get road mist inside them. That's how I've lost a few. Once it has a chance to dry out and the battery has a good enough charge to start her back up, she'll probably run fine until she gets wet again.
Your lights got dim and truck was running sluggish because you were running completely off the battery and you were running it down. Just letting it sit overnight may be enough for it to recharge itself. If the battery is more than 2-3 years old, I'd replace it too. Running on it the way that you were can completely ruin it and I wouldn't trust it going into winter.