Neither. The cables only need to be adjusted if the cable is too slack and you are having problems extending or retracting the slides. They are NOT for height adjustment of the slide itself.
Putting a jack under the slide is potentially dangerous. If your trailer should lose its support for any reason, the jack will cause sever damage to the slide.
If it were me, I would have the dealer look at it and have a certified technician adjust the slide for height. More than likely, they will do nothing and say the slight dip is normal.
Ron