To me that looks like it's the actual clear coat that's fading out.
IMO there's a few ways to tacle this depending on your budget and what you want. (this is not a professional opinion, i'm just bored)
1 - If you want to keep it "stock" looking you might need to have a pro clean it up right and hopefully just a fresh clear-coat will do it.
2 - If you want stock, but less expensive: you can clean them up real good (remove all wax and contaminates) and set up your own DIY spray booth and do a 2 part clear coat:
Amazon.com: Spraymax 3680061 2K Clear: Automotive
3 - If you're off-roading you'll likely scratch the fenders at some point anyway so you can do a number of things: Leave them, Wrap them, or do a bed-liner.
Personally I'd start with a deep cleaning and see if a good waxing/polishing will do it.