We lived in our 32' 5th wheel with a 13' slide for one winter while our house was built.
I cut up several RR ties into 3' long pieces along with concrete blocks at the four corners for a rock solid unit. I also used mobile home skirting to help keep the cold out.
I would never use a jack under a slide out. In fact my current 5er mfg says not too.
Screw jacks/scissor jacks did nothing for eliminating side to side movement. The best they did was eliminating tire/suspension bounce.
If your not going to set up permanently (blocked unit) then IMO a system like the JTs works best.