Cardinal RV Club

We purchased a used 2010 3450 RL recently replacing our old NUWA and I immediately registered in this forum. We are learning the rig now for a trip. I have not studied where someone has posted something similar, so here goes. The previous owner was, shall we say not as concerned about RV maintenance protocol, and did not have answers to many questions.
I was very glad to see the flush out for the black tank. I connected a hose to the flush out , opened valve to drain the black tank, turned the water on, and low and behold, no additional flushing water? What? Somehow the flushing water is not getting into the black tank. I'm new to this model but I have seen friends use the flush out before on their rigs without trouble. No where can I find a schematic of the water system. One look in the storage area trap door around the hot water tank one sees water lines going every where. The flush line disappears behind the Hot water tank out of site? Hmm, is there a shut off somewhere I am missing. I did notice shutoff valves I believe to by pass the hot water tank. Thanks for any help.

Usually the water line goes to a anti siphon valve that is mounted fairly high and then to the tank. Most of the time when they do not work it is because the anti siphon valve is stuck. Sometimes it can be loosened by turning water on and off several times. It just depends on how stuck.

Thank you for the assist. I'll attempt your suggestion.

Evidently this valve is really stuck. I used my compressor with blow out hook up and up 35 psi and it would not allow any air flow.
The flush out itself has a screen covering the opening, so I fail to see any logic in an anti siphon valve. If I find it there may be a an antisiphonectomy and replacement connector installed.

Check all your cabinets and floors in and around the toilet. They tend to bury/hide that anti-syphon check valve in a wall higher than the toilet. If it fails it can cause a flood in surrounding cabinet, wall or some other undesired area. Getting to it may be major surgery. Some trailers have it behind the shower stall. Read the forums. Folks have dealt with this and found them in weird places. I assume you have no water flowing out under the trailer underbelly. That screen on the inlet is likely a washer/screen combo which can pry out easily. I can see my inlet connection from the back side, along with the hose until it disappears in the framework underneath. Mine is in the skirting near the drain valve handles, not in a utility bay. ('07 model) Good luck.