Remis Window surround

The black surround of the Remis over cab window looks terrible. I have washed it using normal car wash several times but it has lost it shine. A very experienced motor homer told me to clean the van with a mixture of distilled water and a cap of Mur polish but this doesn't have the desired effect on the black surround.
How do you do it?

From previous advice to restoring shine, is to use black shoe polish. The principal being it's the shine from the polish (as like cleaning shoes) and not trying to restore it to the actual GRP. If there was such a thing as actual black coloured marine or auto polish, then it would certainly be more specific to restore the black IMO. The shoe polish applied to our over cab last October, does need doing again.