Yes they are, the instructions advise you add some weight to the tanks, you will only need to do this if you want to display with the cab tilted.
Here is my build,
SONY DSC by richellis1978, on Flickr
And some detail shots of the real deal
http://www.britmodeller.com/forums/index.php?/topic/234921843-iveco-stralis-6-x-2-tractor-unit/