The line dictating the skin to use is in the Roster .cfg file. The tga should be in the .dat file. I think it is called a material?
I have only linked tga's to small things that reside in the .dats like torpedo's and bombs. Linking torpedo's and bombs to an object requires linking it as a material if using S3D.
I don't know if any of that information helps, i'm pretty noobish at all this. But I love the model of the sub, be great to see it working. Good luck