I agree with BarneyS regarding the Flushking. We've used one for years, and they do exactly what they are designed to do.
The gate valve shuts off the exit to the sewer, and the hose attachment forces water under pressure back into the drain pipe, and then back into your black tank. If mine broke today, I would have another one ordered by tomorrow.
If you know anyone with a plumbers camera it could save you a lot of guessing. If you could somehow get really hot water into the black tank that would also speed up the process.
I'm still trying to figure out how a Flushking is snake oil...