I always level the trailer with slides in, then jacks down, then slides open in this order:
We open slides in order by size. We have 3 slides. Bedroom closet slide is first (it's the smallest). Living room door side is second. It's the next size up. Back side living room third. It's the largest.
My dealer "recommended" we level first and put the jacks down second, before opening the slides. This reduces stress on the slide mechanisms. I've done it this way, and never deviated. When pulling the slides out while hitch up, I still lower the jacks on the end of the camper, and often will lower the front jacks to, (still hitched), so the trailer is stable before extending the slides. It's just become our way of life.
About trailer lean?
If you park your camper on a level surface , (trailer or motor home, this works for both), extend all the slides and then do a few measurements and determine how much lean the camper has.
Now, pull the slides in so you can move the camper, and this time but boards under the tires to make up the difference in the lean. When you extend the slides, the trailer will be level.
Now, if you want to install a bubble level attached to your camper. Install the bubble level with the tires on the boards, but slides in. This is your camper's level spot and when slides are extracted, the weight of the slides will cause the camper to be physically level, the bubble will be off, but the camper will be level.
(Front to back leveling makes no difference, slides in or out.)
