Go to the Portal tab under the big Air Forums logo near the top of the page. Scroll down keeping yours eyes to the right and you will see a link to the Trailer Inspector's Checklist and a utility to help find volunteer inspectors. Download the checklist, and you will have a very thorough list of things to look at.
Now, if you were looking at a trailer that is in "original condition," then you would be keeping a keen eye out for things that disintegrate over 50 years. Since this trailer appears to have been extensively renovated, you have to look for both things that are disintegrating over the last 50 years, and the quality of the repairs. It is easy to make things look nice, and to cover up issues. Quite another thing to do repairs the "right" way.
How do you know what "good" looks like? I would suggest having a look at the "Full Monty" threads, which describe the complete rebuilds. There is a single thread where the full monties are all compiled. The search function is your friend.
good luck!
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