If you have yellowed areas, it is possible the gel coat is worn thru. If this is true, the best you can hope for is shiny.
If the yellow is dirt, fading and general neglect, you can probably restore the white shine.
My camper had black dots all over it from artillery fungus. Nothing removed them until i tried 'Fast Orange Hand Cleaner with Pumice'. The black spots are (mostly - I need to hit a few more) gone, but the trailer still needs a good wax job to completely restore the shine.