Aside from physical use, how does age alone deteriorate a trailer?

Is it possible that an older trailer which has been stored either in a garage or a barn and has not been used much could be in "like new" condition?  What about rubber bits and pieces (would these be anywhere other than around windows & doors?)  Thanks!

There are a few variables to consider. One is the area in which you live. A salty, humid environment will destroy most metal objects in time. Most older steel trailers weren't built using the zinc based alloys that help prevent rust. Many weren't even primed before the final paint was applied. The enamel paints of their day, are quite poor compared to the newer acrylics and polyurethanes used today.

The aluminum trailers will fare better with age than the steel. They will turn greyish with age, the result of oxidation that protects the metal from corrosion. This can be readily cleaned and the trailer will look like new.

Mechanically, things will deteriorate when unused for longer storage periods. The grease in bearings will slough off the high areas to the lower points. The exposed metal will then begin to rust. The wheel bearings and races will usually have to be replaced. The brake drums will rust, and if badly pitted, they may not fall into tolerances by turning and will also have to be replaced.

As you mentioned, the rubber and plastics deteriorate. These would include the wiring insulation, window and door seals, and of course tires. The bulb bases used in the older trailers will corrode and should be replaced when rewiring.

I purchased an automobile that was stored for 27 years. The body and frame rust issues were minimal, but most of the mechanicals involved complete rebuilds. A great deal of damage can occur, when a mechanical part is unused for lengthy periods.

Trailers are relatively simple, mechanically. Replacing the parts isn't that expensive, most of the costs are due to high labour costs. If you have basic mechanical skills, most duties are well within the realm of a DIYer.

Depending on the length of the storage, will depend on how much attention the trailer will need to put back into service. It may indeed look like new, but need work regardless. Very few will be "like new"