There is a similar join in my Bailey awning rail rubbers, partly because the rail can be removed one half at a time making it a bit easier to handle if like me you had a leak. My joint though is a butt-join, not a gap, however, on inspection of your last photo, it seems to me that the roof rubber has been off at some time and replaced upside down. If it was me, I'd correct it and lose the big gap. Not a big job plus you can have a look at the screws underneath for missing or rusty ones.
Dave
------------- Give a man a fish and he will eat for a day,
Teach a man to fish and you can get rid of him for a whole weekend.