When the slide is open and you place a straight edge along the outer flange is it still bowed?
Most slides are not designed to fit tight to the outer wall just close so the the flange gasket is compressed at least 50% which does leave some room between the slide flange and the wall. It may be just the way the slide is adjusted when closed and the gasket holding the flange out.
Jake my sidekick (yellow Lab)
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho