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.
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.