Since you've indicated the key is in the ignition and the steering wheel is unlocked, there aren't many other potential causes.
About the only solution I would suggest at this point is to have someone ride in the truck while it is being towed and have that person try and turn the wheel in the correct direction when you make a turn. If the truck's wheels turn in the direction your coach turned, then you'll know it is a "following" problem. That is, the suspension and steering system in the truck won't respond to being pulled around a corner. If that's the case, it is a basic design flaw that probably can't be changed.
On the other hand, the person in the truck may be able to tell right away what the problem is and it could be a simple thing like not leaving the key in the right position.
