I'm sure one of our A-Frame members will chime-in with a more detailed plan for you but from your photo and short description, I would think you're looking at taking that end off so you can replace any water damaged wood.
if you are not skilled to disassemble and repair you might call Midwest A-Frame Repair and Service. John Pheil has a long history with repair of Aframe trailers. Just Google for the number.
I would be extremely careful about getting within 18" of that corner. IF, this was an Aliner, There is a coiled spring with a leg mounted in that corner that assists in raising the roof. I've seen photos of them kick thru the rear metal paneling. Dnodoz give good advise.
Hello !
I just did mine , look how bad it was. I added an 1/8 thick aluminium sheet
I had to replace both side walls as they were rotten ( IMG 7823 )
Overall it took 40-50 hours for 400 $ ish of material .