I think Tom's got that right.? On the driver's side, the vertical orientation of the steering column would require a complete re-engineering of the airbag and/or steering wheel mechanism.
While I'm a big proponent of air bags, having been around them since their inception in the auto industry (and I was an airbag test dummy in 1987--for real in a demonstration before a class), I also suspect that a study of motor coach deaths would probably turn up the fact that they're statistically pretty safe.? You're above most auto-coach accidents, and air bags don't help in roll overs or if the passenger is making a peanut butter sandwich in the galley.? They also seem to be driven in a generally safe manner.? Owners are generally adults and have a healthy respect for 30,000 lb. at 60 mph and basic physics.? I would guess that the numbers of fatalities is low enough to avoid the attention of the NHTSA or Congress.
But half of all 18 wheeler fatalities involve unbelted occupants.? The seat belt is still the number one passive safety device in vehicles.