If the car is on a ramp with all 4 tires on a firm surface, the air suspension need not be turned off. You should turn it off only when any tire is suspended like when you jack up any corner of the car. Just my opinion... what do the others think?
If the car is being fastened down with anything other than wheel straps (ie: strap/chains to control arms etc), then the suspension should be off when parked in the truck...