That is not a black tank fitting but it could be used to back flush,, Would be messy, but it could be done.
On my coach there are three tanks ,< Black and gray, plumbed like you expect with a single 3" outlet, and a "Wash" tank that is rather distant and has it's own 3" outlet.
I use the adapter you pictured to drain it, via a 3/4 inch GRAY garden hose..
Back to the hole in the ground.. The 3" elbow has a male hose fitting molded in,, the fitting is blocked but I drilled it out,, that's where the other end of the hose hooks up.
You need a female-famale connector too (Your washing machine hose works well) mine is located at the adapt-a-cap,, Same as in your photo.
