There are two 40models of that year, so this may be the wrongmodel as I looked at the FD which may have varied slightly if you have the 40 KD???
But there are often only small differences and I suspect this will work out okay for you as it is kind of a short trip from inlet to pump and coming back out!~
I did a snip of the plumbing for the FD model here and might let you take a quick look to see if it looks right.
Click this snip to get a better view or go direct to the drawings?
This is the drawing if I am correct on model:
Sheet 3 is the area of concern?
I marked this snipto show what I "think" is happening. The drawing shows the back side of the connections with the water input being on the far right when looking at the back. The blue line traces where hose water comes in and goes to the RV, while it also goes through a TEE connecting to the pump.
As long as you have pressure waterat the input water flows as the blue line and goes to all the parts of the rV which I did not trace out.
But if the hose is off and the pump runs, theconnection is still there and ther is meant to be a small fragile plastic disk at the input where a spring presses it against the inside of the port to act as a simple check valve to stop water going out the port. If that little disk fails to close, the water comes from the tank to the pump and can go out as the yellow line indicates.
Sometimes it is as easy as looking in the port to see the little disk is not laying flat and we can flip it gently with a pencil to get it flipped down to close again OR it may have simply broken in half as it is thin plastic and that can require a new port being put on !
Pump never builds the pressure required to shut down!
So I'm doing a fair amount of guessing there and it needs more checking on your end???