In the past month you have had more problems than anyone else here.
I have helped answer many of your questions but you keep finding more problems.
What is your experience with trailers or motorhomes would help answer your questions better.
Don't know what your style is but do you fill the blank tank full before you dump?
Do you leave the blank tank valve open so it drains all the time or wait until it fills up all the way before dumping?
I would fill it up with the flush hose a few times and see what happens.
If your gauge says 3/4 it may be just paper on that end of the tanks making wet contact will the probes.
Have you looked down the toilet hole to see where the water level is?
If its 3/4 full it should be a few inches above the pipe end in the tank.
With all water off take a flashlight and look down the hole.
Watch this video on YouTube.
That is all I can suggest at the moment.
See you in Goshen at my Georgetown seminar.