The spec sheet states 44 gal for the black tank. So I would doubt you have more then one.
If you don't get a definite answer from somebody who owns one, a simple way to check out what tanks the valves are for is to run water down each sink, shower, tub, or toilet individually and see where it exits in regard to the valves. If you find that they labeled wrong, then relabel them.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow