We don't get much in the way of drawings on the trailer group and that leaves us doing lots more guessing!
So it sounds like you are guessing right but we can't post up much info from drawings for your trailer.
Perhaps I can show some info from the motorized group to let you consider where you might find your problem?
Click these snips to get better view! Just view of random RV and what they did on them!
Just the flush line water comes in and the valve should be closed. Then when you turn water supply off, they want to be sure waste is not sucked from the tank back into the drinking water. The valve should open to break the vacuum/siphon created.
But if it sticks open, flush water comes out like you mention!
I can only say I expect them to be hidden in a wall and to keep lines shorter, they are often close to the flush water input???
Can you locate a space somewhere near the port for flush water and maybe hidden under something like a vanity but up high enough to avoid water just flowing out?
This one shows like I have seen but whether this is always true is an open question. It may be in a wall?
This is what I have seen in limited experience with the flushing system but maybe loo for a space high up under some cabinet where they just left an opening?
Sorry! No real good info! Any chance of hearing the water spray out if the RV is quiet and you listen along the suspect wall?
EXspurt?
That's a drip after the pressure is gone?