Paul,
the perfectionists will recommend GRP rejuvenation cream and compounds used on boats with long winded cut and polish instructions.
The method I have used is a good was followed by T-Cut or similar, when a combined polish/wax. This should cut through the grime, get rid of the chalkiness and leave you with a ice shiny finish. Admittedly it will not be the "wet look", but it will look good. Follow this with a polish/wax at the start and end of each season and you should be happy.
As regards the fridge vents, these are standard parts and readily available from caravan accessory shops. You could start by taking the old ones off and soaking in bleach water prior to a good scrub.


Colin