You said they were having a hard time finding someone to tow you. I can only guess that is the reason they asked you to drive the 20 miles. Personally I would of told them we were broke down and that the truck couldn't be moved.
As far as the location to where they want to tow you, depends on the coverage you have. Unless you have Platinum +, they will only tow you to the nearest qualified repair shop. Not necessarily a dealership or a place of your choosing. If you had the Platinum +, then you could be towed anywhere up to 100 miles.