That airway is over a gulf yes. However I can see them on the RB once they are past a certain point (more north along the airway) and I can see them visually to. Its just that I can see them well south of when the RB displays it. If I can see them visually why wont the RB? Well it would just later along the route, about 10-15 minutes later.
No transmitters in that area that I know of.