This particular manufacturer preferred that we follow the “slides out first and in last” motto. In other words, never move slides in or out unless the air system is in Travel Mode, or in theory straight up or down in relation to that; in that configuration the wall openings are square with the slide boxes, making for less stress on mechanisms or risk to structure.
Leveling the coach, with air or hydraulics, on an off-level campsite is fine, but introduces a twisted relationship between chassis and body, leaving wall openings cattywampous. Moving a square slide box through an off-square opening is ill-advised. This theory has detractors and is controversial among owners, and some other manufacturers tout opposite notions. Following manufacturer guidelines, though, is usually best, and among us has a proven record.
Joel 😉