I meticulously maintain our 5th wheel. I wax it at least twice per year with a good quality wax. It didn't seem to make any difference. Maybe it slowed the process a little but I'm not sure.
I'm frustrated with the appearance of our almost new 5th wheel. Our next 5th wheel won't have a dark colored cap regardless of make. I would recommend that anyone that experiences cap fade report it to your dealer immediately. I would take pictures and document it with an email to your dealer. I know many have been covered well outside the warranty.
Maybe if you have a good dealer and/or manufacturer you may have better success than I'm experiencing.