There is quite a bit of chatter about this topic. RV manufacturers use different installation methods, have different slide sizes & engineering specs, and use different parts. This explains why they have different instructions for whether you put the slides out first or level first.
Since they built the coach, I defer to their instructions. Otherwise if it's a new coach, warranties can be invalidated, but even if it's an older coach, it makes sense to use it as the builder intended.
Seek the builder's recommendations printed in manuals, online, or available by phone if necessary. If the builder is no longer in business, then I think the question becomes specific to the year, make and model, and hopefully someone on this forum or elsewhere can find the recommended text in their manuals for anyone who is not fortunate enough to have manuals.
2013 Fleetwood Discovery 40G, Cummins ISL,Freightliner Chassis, Allison 3000.
2009 Jeep Wrangler Unlimited Toad w/ Blue Ox Aventa tow bar and M&G Braking System