San i flush not working

[dsamford]

2014 Providence 42P
Help is appreciated…
Looking for some past experience or knowledge.
We have a macerator toilet that the green/yellow/red indicator lights always seem to show red in a short period of time.
OK …so just go dump the tank.
For some reason, currently the red light stays on all the time.
I know we can override it and still flush the commode. But it’s a “hold the button” nine second nuisance. 
So I started to use the Sani flush option to rinse the inside of the black tank thinking the level sensors are “covered.”
Upon trying to back flush using the outside Sani flush port, no (minimal) water goes into the black tank.
I can leave the tank valve open and see a trickle of water come through the 4” output drain. (I have a clear 90)
So thinking that possibly there is a check valve in the line I have removed the utility bay compartment panel and traced the SS braided line into a blue PEX back flush line up into the wall.(see pic)
It appears this line after it leaves the utility Bay (yellow end) goes up into the bus sidewall (red arrow) makes the loop and comes back down (blue arrow) and then goes over to enter the system.
So my question is this…..
Has anyone ever had a Sani flush system quit working ?
Has anyone seen this line in the wall kink?
Any other backflow valve in the Sani flush system, if so where?
Am I mistaken in the lines that I see, and this is not a loop that goes up the bus wall?
Any help is appreciated before I cut this line and try and go direct into the tank. Lol

[JohnLoves2Sk]

I flush EVERY time I dump the black. While you drive, things splash around and it's possible that things may have gotten stuck on the nozzle.

There is likely a check valve in the line. I hunted one down in a friends unit, it was in a utility void, it was a vacuum breaker/check valve combo. It was stuck shut after lack of use. You describe it going into a wall then back down - it likely goes up there because a vacuum breaker is up there.
Consider this:
1) If the coach is not level when you use the flush, it may take a while for water level to build up and start flowing out the clear elbow.
2) with a 3/4 full black tank, you can add some cleaning solution and drive around for a while in town. Turns and start/stops should give the top of the tank a bit of a clean.
3) Visit a dump station when it's not busy, connect flush water, and bust out a lawn chair. Leave the drain open, and just let it run for a while. See what happens.
4) Flush for a bit after every tank dump in the future.
5) Find a way to locate that loop in the wall. I doubt it's kinked, but there may be a vacuum break/anti-siphon check valve arrangement in there.

Good luck!