Soft Spot

I noticed in the front left pass through storage area that it was wet after a strong rain. I started poking around and the corner (less than 2" square) is soft. The piece of wood that runs across the camper looks a little damaged at the corner (1"). This is a 2011 keystone summerland bunkhouse. We bought this to keep it a long time and liked the tin and stick because we didn't want the delamination.

First I need to find the leak. I noticed on the corner area that the seam sealer has some holes. Would this do it? the only issue is that the holes are lower than where the top bar is, unless it capillaries up. Any ideas where else to look?

Second, when I find the leak and fix it, do I go ahead and fix the spot or leave it. How hard is it to fix something like this?

Thanks
Bill