That might work, but might not. (Bear in mind, of course, that many vault toilets prohibit the draining of cassettes, let alone black tanks!)
The problem will be at the end of the process. Let's assume you have "milked" as much as you can. There will always be a residue at the upstream end of the hose, since the vault is above the level of the black tank valve.
So when you go to remove the hose from the black tank valve, the chances are really good that there will be a little surprise waiting for you.
Let us know what happens -- good luck!
