just in case if it sounded all doom and gloom - of course not all trailers leak after 30 years; most of them do, however. You maintain it, and it doesn't leak for a while. Then you maintain it again, or it leaks.
If "maintenance" includes seam sealing as early as possible after the purchase, and preferably all those things mentioned in the good link above - then you might not have leaks until time comes to replace the roof (which should probably be counted as part of maintenance). Rubber roof doesn't last longer than 10-15 years, and most trailers have rubber.
All this has little to do with the OP dilemma of aluminum on wood studs VS gelled plywood on aluminum studs. Both kinds have same seams and same rubber roof. The only exception is molded units (no roof seam, no rubber on roof, and no wood in frame), plus recent wood-less laminates like Azdel (there are seams, but no wood in laminate or in frame). Molded units have been time tested, wood-less laminates - not yet, they too recent (and expensive); but these "odd types" wasn't what OP asked about.