To make sure how your RV is setup, get some different food colorings. Mix in a 5 Gal. pail, dump in one of the drains. Then pull the dump valve and see what color comes out. Hint, write down what color you dumped in what drain, easy to get confused. Then take a label maker and label which valve operates which valve.
There is no guarantee each rig is built the same, even if these come off the line one behind the other.
On our Road Warrior, the black dump valve is first toward the front, but that does not mean your RV is that way.