I was driving down the road and my truck died. No power to any part of the truck. I checked my battery cables and the positive cable had melted down by the starter. The vehicle is dead. Any ideas?

Check the battery, sounds like you shorted it out. Determine the cause as to why the cable shorted out at the starter. Replace cable.

I changed the cable and battery and still have no power to the truck.

Vehicles equipped with a pcm (electronic) to not respond well to short circuits or even a reverse of polarity. You could have damaged the PCM which will need to be replaced and reprogrammed. Outside your ability to repair. One thing you can do is open up the fuse box, remove the cover for the fuses and look for damaged wires. The short could have fried the fuse box. Use a multi meter, turn the key to "on" and check to make sure you are getting voltage entering the box. If so, then remove the cover and check for damage.