Rotten wood can be fixed. Whether it is worth fixing depends on how much work and cost you think is reasonable.
Since you have already bought it - and assuming there is no warranty nor ability to return it and get your $ back - then it seems like you will not be out anything extra to investigate and see how bad the problem is so you can evaluate what you want to do.
Hopefully, your trailer is aluminum sided? If so, that will make the repairs easier to do as you can remove the siding in the affected areas, replace the rotten wood, and replace the siding. Which all reads easier than it would be to do. Don't forget to check out the roof.
Good luck, and if you decide to proceed with repairs please post back with pictures and narrative. And also questions, someone here will know the answers.
Here is a long repair thread (clicky)
which may give you inspiration.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch