Water intrusion is the big killer of RVs and Trailers. If you do not know what to look for, hire someone independent to check it for you. If you talk to someone selling a trailer and they start telling you/discussing how much they clean/inspect the roof, that is a GOOD thing! If they give you a blank stare or just tell you they have not had any leaks, look out! FYI, almost all extended warranties have verbiage that lets them off the hook when it comes to water damage. Ditto for the roof manufacturers.