Joe & Jon are correct in their description of air suspension operation, I have customers who have modified their air suspension by welding a chain between the axle and a crossmember to limit downward travel to something less than shock length. I sell dozens of shocks every year to those who don't do this. Suspension manufacturers rely on the weight of the vehicle to keep the bags within their operational lengths when the tires are on the ground. I doubt we could see total bag travel unless trying to level the coach on very uneven terrain. Empty trailers as Joe described, especially those with spread axle suspensions will often lift an axle completely off the ground under certain conditions.