Well, with the help of a call to the service manager at the dealer I bought it from, I figured out where it goes and how to put it back where it belongs.
Turns out it is a seal that is attached by a track to the inside surface of the outer wall of the camper just above the slideout hole. Mainly to cut air infiltration when the slide is fully extended and not a rain seal. Was fastened at both ends by a screw into the wall. By pulling in the slide about 4", I was able to reach the screws and undo them. Pulled the D gasket inside and was able to slide it down the mating track along the top of the slide opening and then replaced the screws. Operated the slide a couple of times and it seemed to stay in place. Hope it continues to stay. Noticed that the one end of the track was slightly deformed, so hope it continues to hold. Very tight area to work in, so would hate to have to replace the track.