Slideouts are designed to support their own weight. If your trailer settles even a little bit, and your slide jacks do not give, something else will. I would never put jacks under my slideouts.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.