Buying the truck first is usually not a good idea. You may get to the RV dealer's lot and fall in love with a TT that your new truck is not up for. If you decide you want a 30 footer, you may find that the truck has too little payload capacity to carry the hitch weight of the trailer AND you, your family, and whatever cargo you wish to put in the truck. Pay no attention to the towing capacity on a half ton truck, you will run out of payload before ever reaching it.
That said, get the most heavy duty towing package they offer.