Under normal circumstances you will probably never need to use the air dump. The air dump will dump all of the air out of the suspension and usually some from the air tank that supplies the suspension.Since dumping the air from the suspension will lower the coach all of the way down to the axles, it should never be done when driving unless it is an emergency situation to clear an overhead obstacle because it can be dangerous.
If your coach has an automatic hydraulic leveling system it will dump the suspension air before it extends the jacks to level the coach. If you have an automatic air leveling system, it does not dump the air, but it will deflate the air bags at the highest point to level the coach. If completely deflating the high side air bags do not level the coach, it will add more air to the other air bags until the coach is level.
Gerald