I was wondering if it was possible to paint the plastics on the Avalanche
Short answer is "Yes". The long winded bit, and this is what I gather from posts at another site dedicated to Avalanche's, is that the plastics - refered to as 'cladding' must be sanded to remove the texture and then finished with your choice of paint.
Just scuff it, spray on plastic adhesion promoter, plastic sealer, base coat, and then top coat. But Your still stuck with the texture. Don't try sanding it or it will gum up into a big huge mess. Unless you plan on priming it afterwards.