The 27' trailer, full bath is shown as having 30 gallon fresh, 10 gallon wash (grey) water tank, 10 gallon waste (black) water tank according to the specs I have here.
That is a rough estimate, as holding tank cpacities seem to change from one model to another, and sometimes in mid year, not to mention previous owner modifications.
If you have a five gallon container, and the patience, you can fill the tanks, and drain them into the five gallon container, and see how many times you fill the container with each holding tank. It will take a while, but then you will know for sure what your capacities are.
It is crude, but effective.