The front tires can be off the ground with no problems. However the rear tires should always maintain contact. In most cases the parking brake is either at the rear wheels or on the driveshaft. Should the rear tires be off the ground there's nothing other than the jacks to keep the motorhome in place. Should one or more slip there's a good chance the motorhome will move.
If the motorhome moves there's also the chance that one or more of the jacks can get bent. They do have some lateral movement to compensate for uneven ground, but there's no provision for longitudinal movement. The motorhome could move forward or backward an imperceptible amount and still bend a jack cylinder or its mount.