My first thought is condensate from the A/C is making its way into the affected areas. Is the A/C unit right in the neighborhood? IF so the fix may well be removing the A/C unit and gaining access to that area from the roof.
The opportunity to spray a mold killer may then exist.
Unless you find and stop the moisture you are fighting a losing battle.
The best way to deal with this is heat treating the area but it requires about 150 degrees be maintained in the rig. Are you perhaps near the desert? A few days in the Arizona desert with the rig closed ought to kill all the mold. IN addition the low humidity will help.
But unless you find the moisture source you are just temporarily fixing the problem