Do you have a flush hose connection on your black tank. If not, add one, then it is not as critical to completely empty, since you can flush until clean. I had that added to the Gemini, when I bought it. The grey tank on every motorhome I have had, has been very slow to drain, for some reason. I just give it that extra time to drain and don't really worry about it.
If you are hooked to city water and once you have emptied black tank, you can shut outside valve off and run more water into the black tank following with another flush from the valves. This helps clean it up a little but also dilutes what little bit of "muddy water" is always left behind. Always make sure the valves are closed before unhooking hose...JIC.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie