I don't know of one specifically. If none is suggested, I'd look for an epoxy type filler that can be colored and start my search at a good local hobby shop that carries supplies for making models (the little tiny kind, not the strutting kind) or the catalog/internet outfit called Micro-Mark
. If that fails, then I'd use JB-Weld or some similar filling type epoxy, taking care to clean any excess away immediately, then after it hardens, paint it to match usig model paint. You should be able to mix a color that would be fairly unobtrusive.
Another good way of removing unsightly stains is trade for a new APEX, which also works if your coach needs washing.