Longer rigs have more frame flex. Schwinteks seem to be sensitive to that. I've found with my rig that as long as I'm parked on relatively level ground, the slides work just fine.
If I'm overnighting in a WM or some such and the parking lot is not level, I always park with the nose up (our bed is N/S) and if it's really bad, I drop the rear stabilizers and maybe raise the back an inch or two.
If it's really REALLY bad, I'll drop a plank or two to level side to side before opening slides.
Oh yes, don't forget that with a slide part way out, it's not sealed!!