the mixer is where the carbuetor normally is on a gasoline powered engine. It is basically just a air/fuel chamber mounted on top of the intake, it contains a metering valve and diaphragm to distribute lp gas into the engine at the correct mixture.
That 'large unit' mounted to the frame is the evaporator/regulator.
If you are getting this smell does it smell like raw LP gas? It could be the regulator leaking. Usually if it is leaking you can smell the odor strongest around it. Of course the best way to locate a leak is to use a gas sniffer analyzer. :o)