Someone here kept mentioning FINESSE-IT a 3M product. Absolutely amazing. I have always liked 3M detailing and paint products, I used to use IMPERIAL FINISHING COMPOUND, so it was a natural to try the Finess-it polishing compound. It's a bit less aggressive and buffs up to a shine easier.
I also found a resin type sealer called TERMINATOR from Top of the Line Detail Products (
www.topoftheline.com) which seals and shines exceedingly well and holds up to weather better than just wax. Top it off with carnuba wax if you want the brightest shine on earth. I don't bother with wax on the RV, just the cars. I haven't had tobuff out the boat yet, just keep it waxed with 3M boat wax.
I just did the sun-side of my Winnebago. I polished it last year and it was oxidizing again. This time I used these two products and I was pretty amazed at how easy it was and how shiney it came up. The front cap was pretty chalky, I imagine similar to your house-top - it's 5 years old this year. Grady gel coat is very thick, so polish that oxidation off and it will shine again.
Scrub it down with dish soap first, will probably take a lot of the chalk off and clean it up good before you polish. Use a wool wheel, be careful not to burn any edges. You need a 2500rpm polisher. WEN makes one that's pretty inexpensive compared to the others, under $100. Here's the WEN version:
You can order one direct, 1000-3000variable speed. You can't use a grinder, they spin 8-10,000rpm.
Top of the Line has the wool wheels, and you need a polishing pad, or you can get them at an automotive paint shop: