Most of these trucks have crummy mileage...

I wouldn't worry about the 12 Code. It just read as a "Start of Codes" and the 55 is the "End of Codes."
Most EGR codes are caused by a dirty EGR Valve that isn't doing it's job quite right because it has too much carbon built up.
You can easily clean it up using Carb cleaner, if you'd like.
If you really feel like changing the valve is costs about $95 at your local AutoZone.