The two shouts you will hear are going to be Fenwicks and Care-a-van. If it is really mucky I would go for Mr Muscle kitchen cleaner with bleach to shift the worst of it first as this will really cut through the green slime quite effortlessly. It will dull the shine so something is needed afterwards to restore the shine, but if you are looking to restore something that bad then there is probably no shine there anyway.
By the way, please turn your Caps Lock off. Some people will see this as shouting and take offence. It's just a bit of "netiquette"
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when I got my caravan it had been stood under a tree for 2 years and was had a thick green coating. A bucket of soapy water with a little bleach added and an old sweeping brush and it came up sparkling