I talked with Clarke Manufacturing out of Mulino, OR yesterday. They're working on a 3.5 gal tank for the RFS KTM's. They couldn't get an RFS bike to use as a model for until recently. They are now working on developing a tank mold using the bike they have for modeling their design. They said that it may still be a couple of months until it is available for sale.
I'm not happy with the range on my 400 E/XC either and definitely need more than 60 miles per tank. I'm hoping that the model they come up with uses the stock KTM shrouds and is clear like the stock tank (with the extra gallon of capacity).