I added the Quadra Bigfoot system to our 2013 Sunseeker 3100.
Both Forest River and Quadra say jacks then slide.
Makes sense that the slide would operate better if the unit is level. Less chance for something in the slide to bind, get messed up, etc.
I think the other point of view is the slide throws off the balance so level after the slide.
As for me, I'd rather be slightly unlevel but maintain the proper working order of the slide.
'13 Forest River Sunseeker 3100
'04 Fleetwood Sedona / Reese Mini 350 / Prodigy / BAL
'94 Coleman Cedar