Wow, such a heated debate. Imagine how I feel. The factors to consider:
Ensolite, (might be able to transfer, but doubt it)
Color, (new top, and cupboard would be White)
Joining to to bottom, (doable, but not easy)
Lots of time
Might not be as smooth of a job, (sagging and misshaped)
Color will be mostly matched, but painting
will still be required
Ensolite can likely be saved
As a rough estimate, I am looking at about 20 to 30 feet of cracks to repair. Similar to what is involved in a belly band. Probably more. The temporary supports and forms will take some time.
I am still leaning towards repairing the existing roof. I don't plan to build trailers from new, and the skills I would acquire, from putting on a new top half, would be less useful to me then what I learn from the repair.
After I get a look under the Ensolite, I will have a much better idea of how I am going to proceed. I am sure it will be gross.