I'll attempt, though I don't have your particular model. Based on my experience, what you really have is two gray tanks and one black tank. Based on the layout of your trailer and the stated tank capacity, you've probably got a 38 gallon black tank that covers the toilet towards the front of the trailer. You've also probably have a 38 gallon gray tank for the forward bathroom drains of the sink and shower/tub units. Then, the back tank (you are calling it black, but, is most likely gray) would be a ~25 gallon tank that serves the kitchen for a total listed capacity of 63 gallons of gray water. Could be the other way around, however, they usually allow more tank space for the shower/tub units than the kitchen sink. Seperating out the kitchen from the bathroom is quite common in longer trailers where keeping the infeed pipes as short as possible is advantageous to building and maintaining the infeeds. Shorter infeeds into two seperate tanks leads to fewer clogs and blockages as opposed to a long shallow sloped feed from the kitchen all the way to the front to drain into a single gray tank. My 5th wheel is setup the same way; a gray and a black for the front bathroom area and another gray in the middle (the outflow is right in front of the wheels) for the kitchen.
That's my guess...the only way to test it would be to drain all the tanks and then run water into the kitchen drain and check the drain pipes. You'll probably find that the second "black" tank, is actually a gray tank and services the kitchen only.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)