To caulk or not to caulk
Really like that 2363, almost bought an Al one until Karen saw the fiberglass 2499 at another dealer.
Our corner trim has exactly the same symptoms you describe. I think it is caused by excess putty tape being squeezed out of the seam and then weathering. I doubt very much that the cracks extend underneath where the tape is compressed. Several people here have replaced roof vents and could maybe comment on the condition of the putty tape that is also used under the vent.
Putty tape is used in assembly, not after the fact, which is why CW probably recommended just caulking it. They sell it because you would use it under a new roof vent etc.
RV dealers sell a plastic putty tape tool, or substitue something else that is flexible plastic rather than rigid, that you can use to re-work the edge of squeezed out putty. I think I read this in Sunline's owner's manual as well.
Definitely don't remove the corner trim to replace the putty. At the most I would clean out the excess that is squeezed out on the side and run a very fine bead of clear silicone that just fills the overlap between the corner trim and the fiberglass side. So far I have just left mine as is.
Henry