Best advice I can give you is to run the slide in a foot and put some wood under the corners of the slide and use hydraulic jacks to lift the slide up until it is close to the top of the opening. You may have enough room then to reach the screws with a angle screw driver or even cut the screw head off. If your slide-out needs to be pulled be sure that you research and find good repair shop that has done this repair in the past. Good Luck.
2018 Redwood 3901WB pulled by 2016 Chev 3500 HD DRW. Goodyear G114 tires, Moryde I.S., Kodiak Disc Brakes, Splendide W/D, Tire Minder TPMS, Sleep # King.