I see it this way. All manufacturers have to provide a Manual with a "Procedure" for putting your slides out. If the Manual said to use common sense when deploying slides, there would be a huge outcry by people that it was too confusing. So......manufacturers give you what works best the majority of the time. If your coach says slides first, would you extend them first while the front end was hanging off of a cliff, probably not.
When ALL of these coaches were built, the slides were installed and adjusted with the coach on level ground and aired up, the perfect situation. Generally, I tell people to follow their Manual, but there will be times where "common sense" says, you should level first and then extend, including the occasional placing of a block under one or two tires to an axle out of a large depression.
Newmar actually changed their procedure mid year a few years back. On really level sites, I level the coach and extend slides, against Newmar advice. On really unlevel sites I extend slides while aired up, air down and then level. Something anyone can do is go outside and look at your "reveals", the spacing between the slide and frame. Some are hidden by trim, but you can still look and get a feel for where the trim is. If it's really out of whack, block the coach or move it some to get it closer to square.
Again....COMMON SENSE is the way to go.