venting from mobile home through new metal roof
I don't know about mobile home exhaust vents. But if you're correct (pipe-within-a-pipe), then I would recommend you have both pipes thru the roof. Having a hidden (blind) intake doesn't seem smart from a maintenance or accessiblility standpoint.
Also, I think mobile homes can only have 1 layer of roofing. So plan on a tear-off. I've seen old mobile homes with multiple layers, and the roof sags between the rafters with the extra weight. Looks terrible.
Note: I am not a roofer.