Does anyone use stabilizers under their slide out?

Bad idea. Just because you can buy stuff like this doesn't mean it's okay.

Unless you have significantly reduced the bounce/movement in the TT frame when set up (stab. jacks + screw/bottle jacks + stab. struts + X-chocks), the slide out will move in relation to the TT floor, which it wouldn't have otherwise. A slide out is not designed to undergo this vertical movement differential. Watch the TT from the outside while someone walks up and down the entry stairs and see how much movement there is.