Need some input on rubber roof repair/sealing

Hey there guys, we recently bought a 24' 1997 Vanguard travel trailer. Im new to the whole rv thing so I've come here for some intel on sealing repairs on the rubber roof. The trailer was owned by a freind and has been well taken care of,however he did point out a punky spot in the shower upper corner that I will tear into over the winter. My question really is about the Dicor self leveling lap sealer.

I found a few spots (I will actually probably tackle the whole roof for peace of mind) that need sealing atttention. It would appear buddy used silicon in a couple spots and it barely help there's also the white stuff that kind of looks crusty. Whats the plan of attack when repairing this stuff. the penetrations I will just remove old, surface prep and use the Dicor but Im not sure what to do along the rain gutter and the entire edge of roof to aluminum flashing. Is Dicor the way to go? Is there better stuff on the market? A friend mentioned 3m marine adhesive/bedding 5200 but that stuff seems pretty permanent

Any advice is welcome!
Thanks guys