My plan was to replace the wood in the corners too, but once I started tearing into it there was more rot than expected (I replaced the entire roof). I'd plan to replace the width of the trailer and start by sitting the membrane 2' back. Peel that section of roof back and evaluate what you're dealing with. You can always go further. Also, when I peeled my TPO roof back it destroyed the material, so you may need to purchase some additional material there.
I also agree with west end, if you plan to keep the trailer don't skimp on the repair.
