Any time we have moisture/mold problems in areas like this where we live, we know that we have to do two things. Get the humidity under control and get that spot better insulated somehow.
Check your local lumber store, see if they carry a product like P2000 insulation board. It's very popular up here in Canada. It's available in different thickness, I think as thin as 1/2". It's kind of like a high density close cell styrofoam with a aluminum like surface.
We use a small household dehumidifier, sized for about 400 sq.ft, and set it for about 45.
That solves the problems.
1995 Monaco Dynasty 36'