Meguires polish

I presume you are referring to ABS plastic. My old caravan was a bit dull and, despite copious amounts of Mer and elbow grease, the mirror shine never returned although it looked passably shiny.

From my yachting experience, oxidation of the gel coat of GRP is a common problem.  A fine rubbing compound should do the trick, before applying a good quality wax polish.  Polish alone, won't do it.  I would be very careful, however,  to use a very fine compound because I doubt that the gel coat on caravans will be as thick as on boats. I think you might find that T-Cut is a bit too aggressive, but a visit to any yacht chandlers will provide you with an array of finer alternatives.

Another tip is to avoid using common detergents when washing the van - nearly all contain salt which will very quickly remove the polish you have spent ages applying.  I only ever use good, old fashioned, Stardrops - because that doesn't contain any salt.

I know this isn't a polish but I have been using this for a while I have never used a polish just give the van a good cleaning with this and the sparkle is there.

http://www.cloverchem.co.uk/2013/uk/product_detail.asp?prod_code='403'&range_sub='31'http://www.cloverchem.co.uk/2013/uk/product_detail.asp?prod_code='403'&range_sub='31'