If I'm understanding this: You wish to install a French door set on the backside of the garage in lieu of the typical plastic film doors?
I would definitely not install any glass door set on the rear of the garage. It will be formidable to attach the jambs into the trailer frame and will add a considerable amount of weight to the trailer, all of it on the rear. It may make the 5th untowable.
A better solution may be to modify the existing doors by replacing the film with Lexan or other hard surface plastics. A commercial door installer is the guy I would seek and there is one on this Forum. Bob's your Uncle is his handle.
