Looking at the picture, it looks like the diamond plate was 2 inches too short so it was pieced and sealed with, I believe you said plastic trim.
The diamond tread on our 990 is all one piece on each side. It isn't glued on. It's attached with trim along the bottom and front edges and some small angle aluminum at the back. The top edge butts up to the bottom of the camper wings and is caulked with clear sealer.
The easiest thing to do would be flatten the diamond plate, put a couple of short screw in the top edge and seal it with aluminum duct tape.
If you clean the aluminum area that the tape will be on with acetone and then press the aluminum tape down into the crevices of the tread, it will make a water proof seal. It's used on air conditioning duct work with exposure to weather and last for years.
Another quick fix would be to use aluminum trim like that used on the front cap to rubber roof seam. Install it with short screws and seal with clear caulking. Either way, you won't see it when the TC is on the truck and it will keep dirt and moisture from behind the diamond tread.