Our TT has two slides on the driver side, and one on the curb side. The kitchen slide is the heaviest and on the road side. This would change the side to side level of the trailer about 1 1/2 inches with the slides out vs with the slides in. (And of course the suspension under the trailer is designed to allow such shifts - while driving.)
After several times parking, we finally got the TT leveled exactly as we wanted. After putting the slides in and pulling up the stabilizers - I mounted a big level like you see on fivers set to indicate level, even though the rig is actually 1 inch off level.
Now when I level to that bubble - things are perfect when I put the slides out.