That is exactly where I had the leak (in addition to the 2 lower corners of the window) and pretty much exactly how my seam looked prior to the fix.
After a lot of discussion with RV retailers, Shasta's Warranty Manager, my retailer and much, much internet research I went against Shasta's recommendation to use Silicone (search that and you'll find almost everyone in the know says never, ever put silicone on the roof of an RV) and instead used Dicor 501LSW self leveling lap sealant. This was 100% effective at sealing the seams - we've had multiple hard/heavy rains since and no more leaks.
I also purchased Dicor 533RM-6 sealing tape that I'll cut into 2" strips and use on the other seams when I get a chance to get up there to them as I feel it is just a matter of time before those seams leak as well. I couldn't use the tape on this seam though because of the putty puddle already around the vent pipe. The tape requires a smooth, right angle surface to mate with and I wasn't confident that would be the case.
I'm no caulking expert (and I hate painting too!) so this may not be the prettiest pic you've ever seen, but no more water coming inside makes it all worthwhile and pure beauty in my eyes.
However, and back to the original point, why am I even seeing a seam in this picture on a "one piece" roof?
We shouldn't have to be dealing with these issues if these were built the way Shasta's brochure claimed they were.