The problem is that you have the wrong roof. I downloaded the first skin you mentioned and had the same problem. For that skin (and most of the other skins I’ve come across) to work, you need the Globetrotter XL Cabin, then the paint will appear (it is right at the top of the list). If you have a mod for sideskirts, the author has included them in the repaint, but by default the truck won’t have sideskirts. Remember you still need to apply the paint in the shop for the mod to work.
Hope I helped.