I can only get Luke warm heat what's wrong

Hello, You didn't mention the engine temperature, so I'm assuming that is OK. You have already mostly diagnosed the issue, and are on the right track. The blend door is the flap you mentioned, but there are a couple, and you have to check them for proper operation, and find out which one is not doing its job. This is not a very fun job, but needed if you're going to drive your truck all winter.