Could be from the hinge pin or screws in the hinges too...

Sorry i am going to jump in late on the wood door issues. 

Most companies warranties at this time period did state that products included in the trailer and not produced by them were not covered by the warranty.  If this is the case and the window company went out of business one would think this is on the owner and not the manufacturer on this age of trailer.  Please someone feel free to correct me if i am wrong.

Unless the company furnishes a complete list of products that it has subcontracted to have built for its trailers, and makes a disclaimer as to its lack of warranty, the manufacturer assumes the suitability of the products it uses.

A particular instance is illustrated when you purchase a new truck. The truck has specific warranties offered by its manufacturer. However the tires are usually irrespectively warranted by their own tire manufacturers.

In this window example, Exiss has acted responsibly, while warranting seven year old windows it did not build.

Gard

I would hope that a manufacturer was not mixing alum components with steel fasteners.  That's a NO-NO because of the cross reaction between the metals.

But I'm sure there are components that are steel that probably shouldn't be. Only way you get rust isn't it?