Larry,
Like Steve said, only extend or retract your slide when the coach is setting on the suspension and at proper ride height.
Only dump the suspension air pressure if you have a manual leveling system before leveling. However, if you have an automatic hydraulic leveling system, it will dump the air automatically before extending the jacks. An automatic air leveling system will use air dumping as a means for leveling the coach by dumping the air at the high side of the coach before inflating the low side air bags. The reason for these air dumping procedures is so that the coach is leveled as close to the ground as possible to make entry and exit of the coach as easy as possible.
Gerald