Years ago before cell phone cameras everywhere I was in a campground in PA and saw a motorhome with it's front wheels 3' in the air and the levelers were resting on a stack of cinder blocks. It seemed like a physical impossibility to even have gotten it into that position without a crane. Wish I had a picture of it. I had a pop up at the time and had no idea you could (and certainly didn't think you should!) get a motorhome off the ground like that.
And, FTR, I've always had all 4 jacks down. I've been on some sites that have been so sloped where I've needed blocks under the front or rear wheels and blocks under the jacks in order to level properly. And always had all wheels on the ground.