If they are like the ones in the Motorhome, they are made of Vacuum formed ABS.
The best repair for these is MEK, which is a solvent that will melt the ABS and fuse it together. This is not glueing... more like welding.
I made a slurry of ABS and MEK and keep it in a sealed quart tin, for when its needed. I have both black and white.
In this case(and I have cracks too in my end caps that I have to fix), you will need to take the cap out. Drilling the ends of the cracks is good.
When it is out, you need to Vee a groove or bevel each crack edge, especially the back.
Then using aluminum tape, on the visible side, to line up and hold the cracks in place. and smooth the slurry into the crack. on the back first. once the slurry is dry, you could paint another layer on and apply fiberglass over it using the slurry as resin.
You will end up with a strong repair, but the panel will have to be refinished.
I fixed my cracked and damaged spare wheel cover using this method.
I started with using a cotton bud to wick just MEK into the cracks, wich melts the ABS and glues it together..
In the case of the missing part, I taped it back and front to contain the slurry...
Then I slathered it on, and put a layer of tape on to hold the shape... this took a week to go hard as the tape blocked the solvent evaporation..
Some of the areas around the clamp, are subect to stresses, so I used fiberglass and slurry to reinforce.
Here is was in progress..
Still need to paint the cover, not decided the color yet. I stuck it back on to keep it safe until I am ready.
The cracked and missing sections of my fairing are next!
In closing... its repairable, just trying to show you how!