I doubt that the door from a different trailer will fit as each has a unique shape.
You can take a hint from this thread where a member is reforming the shape of his door by doing some kerf cuts.
Rookie vs. 1990 Casita (Project Thread)
But if you decide that does not work on its own then try this:
I suggest you make a template of the curve from top to bottom and of course the width as well. You will create the template working directly off the trailer to copy its shape. Then use that template to create a set of stringers covered with thin bending plywood to create a mold form onto which you can lay up some fiberglass.
Hopefully you could even reuse some of the old door by cutting it into sections and then add a new layers of fiberglass to bond it back into the correct shape using that form as a backer. You won't know if that can be done until you get the form built and try the original door against it.
But at least it looks like you have a great space to work in that is out of the weather. If you don't have the skills for the project don't worry, a jigsaw and steady hand for form building is the worst of that part of that job. Working with fiberglass is not extremely hard to do either. Patience is required