I have used "Totally Awesome" found in one of your Dollarama stores in the USA. It's less than $2.00 for a liter spray bottle, spray it on a cloth rub it, it will begin to whiten. The original white will come back, I've used it for years on all white plastic, like marker light housings, trims, kitchen exhaust outlet hood, if you do it once a year,[painless really], it will stay looking white.
I'm not sure, have never removed any of ours, but at a couple areas, the aluminum skirting wasn't fastened high enough and the cover strip comes undone and I have to push it back on. I will probably have to do some work to stop this from happening. That leg on the cover may have to be cut off, but that would be easy.
I had the same problem. The skirting pulled out because it wasn't installed high enough under that strip. I pulled the whole thing off and re-drilled the strip and skirt, and screwed it back together. I took a few hours to do it. Be careful of not pulling the cover vinyl all the way off or it will ruin any decals that are under the sealant across the top if that makes any sense.
Craig and Mary
2017 Montana 3820FK
2020 GMC 3500HD