[petrock]

Took it to the dealership and they said the temp sensor had shorted and was replaced. We're a month down the line since they fixed it and it happened again...

If the truck is still under warranty, take it to the dealer and have them fix it (again). What you are describing does sound like a bad temp sensor or the thermotsat is stuck open. I suspect the latter since the heat wasn't hot. Don't just throw parts at the problem because of something you read on the interwebs. Its like throwing darts in the dark and hoping to hit a bullseye. Good luck.