Just an FYI. Best to have someone inside to make sure black tank doesn't get to full when you add water to it as it can overflow. Not sure how that works or if people have faulty tank connections but I've read that often on these forums with nasty leaks to the floors. If you're tanks are REALLY full, such as gray water, and you drive, water can even slosh up into shower floor or bathroom floor.
Not sure I understand this, you said you dumped like normal, grey then black ... isn't it black first and then let the grey help to rinse out the hose?