A very well done Sturmtiger; it's almost like loooking at a photo. The only thing that strikes me as a little bit odd is that the wheels look kinda flat even taking into account the fog. However, as I would never be able to produce anything half this good I feel a bit stupid pointing out something like that.
mhhh...the Mobile workshop seems a lot like the Amrican WW2 Heavy tank named T14,ar the Matilda a british WW2 tank,both without a turret,while the under costruction tank has the lower plate of a pz6 Tiger(if you google Tiger Tank and look at the tracks it`s a lot like your drawing ) and the upper part of a German tank destroyes,a lot like the Jagdtiger/JGpanther...sorry for the NERD like description,well done job,but look at the cannon of the tank,the gun`s shield is too big,so it has no gun depression,by this little thing tehre`s everything commander`s cockpit,drivers`s hole and gunner `s aimer! Had fun observing your painting!Nice work!