Start looking outside from the roof--> down. A misshapen or unsealed roof will denote water damage. That can be a chore to fix. Look for loose or missing screws along the siding. Again, an indicator of water damage and rotting frame. Look underneath for rusted frame, bent axles, at the underfloor of the trailer.
Now, go inside and press against all the paneling in the ceiling and walls. Inspect carefully!
Boles Aero is a well regarded brand. I think they even have a Boles Club for owners. No one on the internet is going to be able to tell you how much work and expense is going to happen with your restoration. FWIW, I spent about 6 months with a complete gut and restoration on a 22' trailer.
If you do decide to keep the Boles and do some work on it, you'll be rewarded with a cute camping rig that will hold it's value for some years. Good luck on your Boles adventure!