The one I had replaced was the one on the upper right hand corner on that picture. It is an overflow, but UV does effect them. Fact I was adding some anti freeze on mine this morning and noticed that UV is starting to flake some of the ridges. So I will probably have to replace it again pretty soon. But the fireman is correct that on a Freightliner it is not the shape of a football.
Everything on my coach is exactly like that picture except my hood is a lot bigger and goes all the way down on the end cap exposing the fuel filter for easy change out.