afterthoughts :

I can think of two potential downsides. I looked into this with a smaller trailer (About a 14' version), but passed (Mostly due to the poor condition of that specific trailer). The tires, axles, and suspension should be the same as a regular trailer. But, the frame rail spacings may be different. This will vary from trailer to trailer, but make sure that the frame rails are close enough. If I remember correctly, an enclosed trailer will be at either 16" or 24" centers.

Also, the walls may not be quite as durable, and will certainly have more spots for potential leaks (And, windows reduce structural integrity). This can be fixed, though. Run plywood and framing boards on the inside of the trailer- you'll get smooth load walls, and it should also beef up the structure of the trailer.

When you're done, take it to a scale. Calculate the GVWR, and subtract from it the unladen weight. This will give you a payload rating (Provided your truck and hitch is rated to tow the initial GVWR of the trailer)

I think it can definitely work. The trailer I was looking at was a 1971, and had fallen apart too much to be usable. If your trailer is in good, sound shape, some elbow grease can build you a very nice hauler (Plus, how many enclosed trailers can have a bunk bed in them)