After flushing the black tank, close it. While flushing the gray you can connect your flush out hose (20' should be plenty) and run some water into the black tank. Most dump stations do have water spicket for this purpose, have seen some with a hose too. Then flush black tank. A clear attachment will allow you to see what is coming out. You probably want to flush, drain and repeat. As said, don't leave your black tank dry, add a few gal water + chemical. Keep your flush out hose with dump equipment. Ours is an old water hose that is wrapped with black tape to indicate dump hose.
Also, you want to flush black tank when it's about 3/4 full so it "whooshes" out. If not that full, add some water into toilet prior to getting to dump station. You want to be efficient/timely as you might have people waiting.
Have gloves ready & equipment easy to access.