Are you sure that is not wear from the cover? Seems strange that even the sticker is showing color change in the same area.
I have Ceramic Coated my truck, and may do my trailer as well. It's cheap for long lasting protection.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
There is enough in this little bottle to do my truck twice, or an entire trailer with leftovers. After that, just a wash and a wipe down with Reload every so often. Waxing these boxes can be such a pain.