Hmm if it disappears when structure all you may have imported some rogue verts somewhere
Copy the main tank into an edit lod , go back to original lod , delete the main tank and then find components see if there is something somewhere making it broken.
You could also try in game to get bounding box return and see if it makes sense for size of tank. Also check binaries log and arma .rpt for sign of. Strange convex component in component 0x .