The best, longest lasting product to "restore" the plastic cladding on Avalanches (and the cladded Avalanches have a ton of plastic on them!) is called Refinish Restorer; you can buy it on eBay. It's good stuff! I only had to apply it 3 times (2 thin coats applied each time) in 14 years of ownership. Initial prep work is the most important step in getting good, long lasting results. I still have the original can of product and there's enough left in it to treat 4 or 5 more cladded Avalanches; I keep the can in my basement to keep it at a constant temperature.
I will be using this product when I need to dress up any faded/aged plastic on my Supercrew.