Neither of my black tank fresh water flush ports seem to be letting fresh water in the tank to clean it out. They just build up pressure and nothing goes in the tank. Then when you unscrew it spews all the fresh water back out. This is on a 2022 BT36 and is the same for front and back bathrooms.
Our new Four Winds 31W came from the factory with this issue. Turned out the anti-siphon/back flow check valve was reversed plumbed - apparently a common issue. Warranty fix at dealer is your best bet unless you have the correct PEX tools and skills. In our 31W, the valve is located inside the wall between the bedroom and bathroom. It's up high in the wall, I'm guessing to mitigate any chance of blackwater infiltration and mitigate any potential siphon effect. Ours was accessible thru the far left cabinet over the bed, left side panel against the wall. If necessary, you can get to the PEX piping leading up to it thru the access panel within left nightstand or to a lesser degree, the access panel under the bathroom sink. I know this because I first tried cutting the lines below and inserting 1/2" PEX couplings to reverse the flow.
After reversing the lines, I tried another flush and water started pouring out of the coach onto the driveway. I discovered the water was coming from inside the wall flooding the bathroom floor! After opening up the side wall of the upper left bedroom cabinet to expose the valve assembly, I discovered someone at the factory had nicked the PEX at the valve with a hole saw