This is a dry bulk pneumatic tanker. I used to work for Koubenec Motor Service (no longer exists) years ago washing these out between loads. The hose on the front is connected to a pto driven positive displacement blower on the tractor and air is blown down the discharge pipe at the bottom, air is also blown up the pipe on the front to pressurize the tank to aid unloading, there is also a pressure relief valve on that pipe. Also if you look at the side view picture, on the bottom of the hoppers there are smaller hoses connected to the bottom cone, air can be directed to these to make them vibrate the tank. They are normally loaded through hatches on the top but there are also versions of these that are hooked up to the intake side of the blower and pull a vacuum to draw product into the tank. The trailers I washed out were used to haul sand, powdered clay, coal powder, coke, plastic pellets, and there were food grade trailers for sugar and corn flower.
Thanks Ron. I have replaced my guess as to how it works with a referral to your comment.