If it is originating from the inside I would think paint may be your only answer. My bigger concern would be what is actually causing this and are you really just ignoring a bigger problem. 
facory wrote:
I have a 2008 FunFinder Xtra Toy Hauler.  White fiberglass exterior.  Last year we had a water leak in the roof at the right rear corner.  It was properly repaired and hasn't leaked since.
As a result of the leak some water must have got into the wall.  Now there is a brown color coming through the exterior.  I figure it is from either cardboard or plywood that is part of the wall construction.  Apparently, it got wet from the leak and the color is coming to the exterior.  I have tried various bleach products to neutralize it, but not working.
Anyone have any ideas for getting rid of this discoloration?
TIA