Matt,
Not sure how much help we would have here given that most of us have Beaver coach experience.
I would call up the RV manufacturer and ask what they used to do that sealing job and if they might recommend the same product or another product to replace the sealant or possibly lap over the cracking. They should have a safe solvent recommendation for cleaning the surfaces before you add any new sealant also.
Beyond that the places that do rubber roof replacement or repairs may have some good experience to call on.
Later Ed