Doesn't sound like a mechanical problem to me, sounds like fuel/electrical, take to another dealer or a good (non dealership) shop and see what they have to say. If you were able to post the codes on here we might be able to pick through them and shoot some suggestions out. There is just a lack of info right now. Could be something as simple as the coils going bad, or egr valve stuck open, they can throw a LOT of codes at one time, especially if it was driven under those conditions for some miles.
Not to knock on your knowledge, but perhaps the dealership saw you as an easy mark, get someone who knows a thing or two about vehicle mechanics, bring it to another shop and let him/her talk to the service people, you'd be amazed what a difference it can make in how they talk to you and what they come up with for a diagnosis.