If you are getting faulty readings on your fuel gauge, I am afraid that it might indeed be the fuel sending unit which in on the fuel pump located inside of the forward fuel tank. If you did indeed run the truck out of gas and then the truck was left with the key in the on position for an extended period of time, you probably did burn up the fuel pump.
Now that you have filled the tank it will be a big time pain to drop the fuel tank to change the fuel pump if that is indeed bad. If your tank is indeed full you just added an approximately an additional 200 lbs of weight to the fuel tank so you will need to drain it before it can be dropped without something to support it. I would take it to a professional that has the correct tools to do the job right.