It does not have the hazardous material identification.
It appears that at some point the rear bottom portion of the tank was cut out (corroded maybe?) and replaced with the flat shiny piece of sheet metal you now see. Only, rather than welding the replacement plate in there is seems to be bolted/riveted with some sort of black sealant (small amount can be seen along the top of the plate). I don't think that would be considered and acceptable repair. That tanks gets in a wreck etc it would only take a very small twist/bend/dent etc to make that start leaking....
That's the only thing I can see in addition to what the others have said.
I was going to guess that it looked like brewing mash that is being transported to use as animal feedLooks like raw sewage coming out of unsecured drain.