As many have said, do what the manufacture says. The reason for the different orders, has to do with the actual frames and slides. Some are very ridge and do not warp on uneven ground. The reason a lot of manufactures want it leveled first is, if the ground is uneven, it could twist the chassis enough to jam the slide(s). If extending the slides 2nd, causes the MH to be unlevelled, nothing prevents you from doing a manual touch to make it level. Cabinet doors and actual doors will also give a good indication of level.