Like others, I'm not really worried about the superficial rust. A few hours with a wire brush, then some good paint will take care of it. I've never used any of the "rust converting" products that you can supposedly spray directly onto a minimally-prepared rusty surface, but they sound too good to be true. I would advise thorough cleaning with a wire brush, right down to clean metal, and proper priming and top coats. I'd be pleased to hear from anyone who has used the converters with good results on surfaces as rusty as we see here.
Regarding the "leak". Let me be a devil's advocate for a moment. First, what part of the TM is shown in your third picture? In my 2006 2720SL, I can't find any place with a vertical trim strip that close to a corner, and it doesn't show on any of the dealer pics, either.
The extra vertical trim strip is poorly applied, and the metal to the left of it is new (different color). The metal to the right of it appears to be stained in a strip 3 or 4 inches wide and parallel to the ceiling (unless that is an artifact of the photo). One explanation might be that the original metal in the corner was so badly and visibly corroded that it had to be removed and replaced. That would imply a big leak into the wall from the outside corner. Since the wall has no meaningful draining or drying mechanism, we would have to assume that it is still wet inside. Does that make a difference? I don't know. In the earlier days of wood framing in the shell, we would immediately worry about rotted wood. But by 2005, the walls were framed with aluminum, so rot is not an issue. Will the inner skin continue to corrode, necessitating further repair? Again, I don't know.
Let me say again that this is pure hypothesis. I have no idea if it is really true. But if I were looking at the unit, I would first request that the extra trim strip be removed. Once it is off, take a look at the edges of the metal skin, looking for ragged corrosion that is covered by the trim. Then stick your finger into the gap to see if the foam inside is wet - and whether a substantial amount of foam is missing. Then go outside, under the lower edge of the shell directly below the area in question. Remove a couple of screws, see if they are rusty. Stick a probe of some kind (the stem of a cotton swab?) up into the screw hole and see what you find.
Let us know what you find.
Bill
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