You should be concerned with the BRAKES/BEARINGS. Would need to tear into them and inspect.....easier to just replace with new
Tires....forget about them. They WILL need to be replaced
After that you would have a trailer that could be moved.....that would allow you to check it frame was twisted from improper support for past 13 yrs
Was roof/side wall chalking maintained or is it the same old original dried up chalking.
Any internal signs of water staining?
Any signs of external 'delamination'?
Do the appliances work?
ETC ETC ETC
