You guys are right. HRC just confirmed for me that the rear tanks are aluminum because they are easy to fabricate in a short time frame.
Also we had some discussion about the blocks on the ends of the swingarms when the bikes were first photographed in Peru. IIRC, HogWild had some awesome explanation about passive traction control devices. Well, I asked and they confirmed that they were nothing more than blocks to allow the use of road race style bike stands. Unobtanium.