I'm assuming you have a removable access panel in the shower. If not, you might leave this to your dealer.
Here's a drawing that shows how it's supposed to be hooked up. The problem with figuring out how yours is hooked up is that it may or may not be easy to determine which pex line is coming from the inlet in the UDC. If you're lucky, the pex lines in and out of the vacuum breaker will be different colors. If that's the case, you can check the color of the pex line coming out of the UDC fitting and see if that's attached to the bottom of the breaker as shown in the pic. You'll have to take down the rear wall of the pass through basement storage next to the UDC in order to see the pex line going up to the breaker. With color coding, the determination should be easy.
If there's no color coding, you could just try swapping the connectors on the vacuum breaker. But if that doesn't fix the problem, you should put the connectors back in the original position.
If installed as in the diagram, the fittings should easily twist off by hand and go back on hand tight. I'd advise that before you disconnect anything, you tie strings to the pex tubing and vacuum breaker so that nothing can drop out of reach while you're working on it.
If it still doesn't work after swapping the lines, you may need to test the pex line that comes from the UDC by carefully running a trickle of water into the UDC fitting with the fitting disconnected from the input side of the breaker. Try to catch the water than comes out. Don't run any more than a trickle. You don't want to find out that there's something underneath that would be damaged by water - like an electrical component.
If water's coming out of the pex line, put the fitting back on, and then disconnect the fitting from the output side of the breaker to see if water gets through the vacuum breaker.
If water gets through the vacuum breaker but not into the black tank, you'll almost certainly have to let the dealer handle that repair. But that's very unlikely.
Hope this is all clear. Good luck and let us know what you find.
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