If the seal is like mine it slides into a track and is free to move. A small amount of silicone sealant on the end should keep it from sliding.
Hopefully it is only sliding while the slide is extended and not while the slide is retracted and you are driving. If it is sliding while driving I would be concerned about inadequate sealing pressure and the potential for leaks.
Here is a picture of the end profile of the exterior seal and you can see the rail that runs along the track. Thor sent me about 40 feet of exterior seal before they sent me the interior seal which was missing from my coach. I had to send them the picture so they knew which seal not to send.