I hope I'm understanding what you want to do as the post above has me kind of confused.
So you want to connect a short garden hose or traditional RV drinking hose to the shower head threaded arm and then connect the Valterra wand to the other end of the hose. The wand end you want to stick down the toilet.
Am I interpreting that correctly? If so, you can buy an adapter at the local hardware store to do that. This is what I think you'll need but those with more knowledge of plumbing can correct me if I'm wrong:
An adapter that is 3/4" GHT (Garden Hose Thread) on one side and a female 1/2" on the other such as this one:
Dixon, 5071208C, 3/4in Male GHT Adapter x 1/2in Female NPTF
Again, since I'm not a plumber, I'm just assuming that the male 1/2" thread will mate with the female 1/2" NPTF. I'm not sure if the NPTF will leak or not but I assume it will at least screw in and mate property. Since it isn't a permanent connection, I'm going to assume that a slight leaking will be acceptable if there is any at all. And even if there is leaking, it's falling into the shower pan anyway.
So this is what I'd give a try if I were in your shoes: pictures below show connections in numerical order--
eta: after re-reading what Chris wrote, I now understand what he's suggesting to do so you can disregard what I'm saying
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