1. Slides do NOT seal when partially out. That means not just water if it rains but dust and such can easily come in regardless of the wipe seals
2. Schwintek advises against partial extend or retract due to the timing of the 2 motors may get out of time. I do not believe that as I have partially extended or retracted hundreds of Schwintek slides while working on the RV's and never had a problem---see item 3. Also, a Schwintek room when partially out and you are putting weight in the slide box will put extra pressure on the gear sidewall system. The system is designed for that room to be fully extended so the inside flanges are tight against the wall to support the room box. Next time you go 1/2 way out, push on the upper flange on the inside and you will see the room move up and down. NOT good for the Schwintek rack system.
3. Regardless, the CORRECT procedure when operating a Schwintek slide is to HOLD the extend or retract button for 10 to 15 seconds after it fully extends or retracts and THAT gets both motors in time again. ALWAYS do this and your system will operate better.
4. So, if you partially extend in your scenario, just make sure when retracting you hold the retract button for that 10 to 15 seconds.
5. As to leveling jacks and Schwintek systems, I have found that it is BETTER to always extend a Schwintek system BEFORE extending leveling jacks, especially on 5th wheels. ALWAYS retract those leveling jacks before retracting the slide room. Motorhomes rarely have a problem with Schwintek systems, so jacks up or down makes no difference. Towable frames are not as strong as a motorhome frame and the jacks tend to bend the frame at the points where the jacks hit the ground and IF that warp is centered around the Schwintek room, that causes the room to bind just enough to cause extend retract problems. Doug