What you have there is structural frame failure. The failure must be fixed prior to exterior repair.
I had the same problem with my old trailer. It was a used 2005 Keystone Outback 25RSS. I bought it that way without knowing and the dealer was more than happy to not disclose it.
(the previous owners or the dealer covered the cracks with oblong side marker reflectors) The trailer had a manual rear slide with a short queen.
I removed the interior paneling on mine to expose the aluminum frame structure.
I found that it had leaked for a long time before the previous owners traded it in.
The aluminum welds had weakened the connecting aluminum framing and caused the horizontal frame to break away from the vertical frame by about a 1/2" next to the welds.
I repaired it by using "T shaped" steel framing plates, pre-drilled and screwed the frames back together at the corners that broke.
I then washed all the black mold with bleach, replaced the fiberglass insulation and put it back together.
I used a product called Marine-Tex Epoxy Putty
for the exterior repair after I pulled the years worth of silicone and Dicor out of the cracks.
The epoxy dries bright white. Cracked the epoxy every year after that but the frame did not let go again. I covered the exterior, cracked epoxy with Dicor to stop water infiltration.
Fix it best you can and trade it.