If they did check everything in the front end they should have picked up any loose tie rod ends, etc, but did they look for any looseness in the steering box itself. Does your wheel feel tight or does there appear to be a little more "play" in the wheel then before.
I know this seemed to happen on a trip, which means that something changed but have you checked the total weight on your front axle. Not saying that is the problem but I know that most coaches drive better if the front end is loaded close to max axle weight and that a light front end will tend to aggravate any suspension problems.