Used camper purchase, possible roof issue - please advise

Looks like the trailer is kept in indoor storage. That roof looks real clean! That damage also looks as if it was just recent. Have you asked the owner about it? I would look for signs of water damage along the inside corner and floor. You would see some discoloration and possible softness on the wall. It may just be a bad install! As far as the sealant job goes, that looks factory!