The tank flush in mine goes up under the bathroom sink to an air gap back flow preventer, and then down to the black tank. There's no valve or anything to stop it from flowing, once pressure is applied.
Now, my fresh water inlet is in the same compartment in the basement, and it does have a valve to switch between direct city water and filling the fresh tank. Are you sure you're on the flush inlet and not the fresh water or winterization ports?
Best of luck. 😃
2014 Ram 3500 - DRW - 4x4 - 6.7L - 68RFE - B&W Companion
2015 Montana High Country 340BH