You 'think' it sags......use a level like RON did and check it.
Slides are 'self supporting'
Slide out jacks sold aftermarket (cause mfgs do NOT supply them) are just a unnecessary product to get your RV monies like RV Toilet Paper, Anode Rods for Atwood Water Heaters etc.
YES they will support the slide BUT every time you walk inside, move around inside, go in/out of trailer it moves up/down due to suspension/tires.
When moving down those slide supports prevent slide from moving along with rest of trailer.
That lack of movement will damage the slide
Use them if you wish.
But if slide really is sagging get it properly fixed vs band-aiding the symptom
and causing further damage
Wonder why mfg. didn't supply slide supports :H