Originally Posted by TeJay
Putting scissor jacks under the FRAME as close to the mid-way point of your trailer or just in front of the front axle will also help a lot. If you jack up on the AXLE the frame still moves up and down on the suspension. This won't reduce trailer movement. We purchased screw jacks from CW that fit under the slide rails. That helped a lot. Our Flagstaff slide is a dinette and couch slide that is probably 16-20 feet long and it slides out almost 40"'s. It needs some external support and those jacks did the trick.
I plan on adding jacks under the frame near the axles to get rid of some of the excessive shake.
But, I did want to comment- if I'm reading right, you're putting jacks under the slide itself (on the rails); if so, I believe most manufacturers strongly recommend against this practice. The main part of your RV can shift and move where the slide itself doesn't and this can add pressure to places that it's not designed for.