The "Flush King" is not going to work very well, (if at all,) with the drain lines of a Casita
black water tank. They don't flush anything, all they do is refill the tank with water, which could be done for free with your garden hose. None of those purported "flushing" devices or gimmicks located at the end of the discharge pipe will work when the plumbing drain lines form bends. You get a tidal action, not a flushing action. The only way to really flush out a holding tank is to actually put the pressure stream right where it's needed-in the tank, not two or three 90 degree bends away.
The "wand" is ok if you like having to drag a garden hose into the trailer each time. Then, there is also the yukkiness of having to deal with that mess and smell each time you flush the tank. I installed a regular rotating pressure flushing sprayer in my black and grey tanks. The flushing is done with a garden hose from the outside of the trailer, so I can not only flush the tank, but watch the "results" as I flush so I know when it is running clear. Only takes one person to flush, so it is very handy for solo travellers too.
I do use a separate hose I keep just for flushing, but it does have a built-in one way check valve to prevent any water in the line from backing up into the water supply hose as well.