As a former commercial waterproofing contractor, I recommend removing all old sealant. You wouldn't paint over old paint that is starting to peel, would you? I also like to see why things are cracking or leaking.
I use silicone sealants because they will outlast the trailer. I use butyl tape under lights, etc., because it can't fail unless they come loose.
Regardless of what you use, surface preparation is key.
Clean with alcohol and a rag, and wipe dry with a clean rag. Do not let the alcohol dry before you wipe it!
Me, Julie, Lil' Barry, Faith, and OSDs, Fang and Treaty
2003 Hyundai Starex (H1), 2012 Coachmen Clipper 126
I don't know when we'll be able to go camping again...