How old is your RV? I'm wondering what sort of tape the OEM used on your rig...
I put Eternabond Roofing tape on the front and rear cap seams of my '94 Bounder shortly after I bought the rig in '04. More than 12 years later the tape was still doing fine. No evidence of any cracks at all. I also put it on my '02 Winnie shortly after purchase in '16.
I'd recommend using Eternabond tape to replace that cracked tape. Not sure how your RV's OEM did their FG roof edges but if they are anything like the Winnebago setup, you might want to inspect for issues there too. Here's a link to my RV blog's roof maintenance section, maybe you can find something useful there:
Roof Maintenance... I would be very interested in seeing some pictures of the roof edges and end cap seams on your RV.