Some Lippert frames are constructed of thin metal welded together to form an I-beam. That metal will bend easily and a jack, that is not centered perfectly, will roll the edge and will come crashing down. A floor jack will still bend the junk frame, but at least it will stay up. With a frame that has a stronger beam, you might be OK to jack it, but a bottle jack still may slip.
Jacking under an axle can cause the tube to indent if it is under the leaf spring. In that case, a block should be made that contacts the u-bolts and axle tube forming a cradle that spreads the load. I made heavy shock mounts that give me a jacking point that lifts by the u-bolts.