Li, I think I would dump. After 3 days, you should not have to add that much water to make half a tank or more. Not knowing how large a tank you have, hard to say. But I would not let such set in the tank for however long. After dumping the black tank, you could go in and flush your toilet several times, fill the bowl up 2 or 3 times and let it go down, this is before you dump the grey water and you still have the hose connected, of course.
I usually do this if I am heading home and r.v. will be setting in my yard.
Sounds like you have the idea of how its done. I sure would like to be a little bug somewhere watching.
I sure remember my first time, think everyone does
It all went very well, even though I sure had doubts about myself doing anything right. I sure was proud of myself , proud over the fact, I dumped poop without getting any on myself.
Usually they have a water hose there to wash your hose out, before you disconnect it from the drain, so thats nice to use to. Thats where you have to take care you don't get splashed on, from wherever.
Good Luck, but you're going to do just fine. Enjoy your time in your R.V.
CArol