I have the Dometic SeaLand 8100 macerating toilet in my 7 yr. old class b. Never any problem (dreading when I do), but if anything, mine fills too slowly.
Since this model has electronic wall controls, I kind of thought I'd hear some actuator sound when water was requested and when it shut off. But I only recall silent operation. There has to be fresh water valve that is controlled by the wall flush buttons. Was anything like that replaced?
The operations manual trouble-shooting section mentions solutions for my slow filling, but does not mention your problem that the water won't turn off. I believe it has to be the same (an only) fresh water valve that controls both.
I will be following your thread, just in case. Good luck.