I'm not familiar with your particular RV. So I will speculate.
Do you have one or two toilets? If one, I would doubt very much you have two black tanks. If that's the case, it was more then likely miss labeled at the factory. I know--that would never happen.
A way to check it out, is put some water down each sink/drain (one at a time) then pull a dump handle to see if and where it drains. With a little experimenting, you should be able to figure out what you have there.
Let us know what you come up with.