I'd use fiberglass to repair the plastic from the backside, then use a fiberglass reinforced filler like duraglass on the front.
For places where chunks are missing, run masking tape across the front side to form a rough mold and push fiberglass into it from the back.
After that hardens, hit it from the front with duraglass to fill in the gaps.
They have fiberglass repair kits at walmart that include woven mat, resin and hardener.
Watch some youtube tutorials, then start playing with it. It's not hard. Especially for this application.
Being half of a virtual on, my cab had t-molding on one side and the boards to mount the endcap on the other.
The endcap didn't come with it. I ripped off the boards and filled them in with wood putty. I should have used bondo.
I never could get the wood filler to the same texture as the rest of the panel, so it is visible when the light hits it a certain way.
If you have
any experience doing bodywork, fixing the plastic won't be any more hassle than removing the boards and routing a t-molding slot.
The good thing with them is that you can work on them away from the cab, where dust from sanding won't get all over your cab. The same can't be said for routing the t-molding slot.