If it is a stress crack, you can try a plastic cement. Make sure whatever you use is safe for plastic. Don't use cyanoacrylates like super glue. Also, I used a backer piece of plastic for reinforcement.
if it is from oxidation, that is . a different story. When that started to occur on the fender of our last trailer, it essentially marked the beginning of the end for the fender. The technical term I believe is photo-oxidation. Similar to rust, once it starts, difficult to combat. I tried glues and plastic welds. Not much help since the UV breaks down the integrity of the whole fender making it brittle. The cracks are just from the stress of the contact points where it's screwed into the trailer. I'd be interested if anyone has had success with the brittle. One idea i had was to plastic-dip coating the whole thing, but it will also make it significantly heavier hanging on the screws, further stressing the contact points.
What color is your fender..tan?
Just keep in mind the effort to try and fix them. You can replace them for about $200.
2017 Ford F350 Super Duty 6.7 King Ranch Crew
Sold the 2016 Crossroads Sunset Trail Grand Reserve 32BH
2020 Grand Design Momentum 320G