We have a 2020 Ventana 3407 with just one toilet, but we've had a similar problem. Sometimes when we pull over at a rest stop the red "tank full" indicator is on, and the toilet will not flush. This happens more often if the coach is not on a level surface, and I know that the tank is nowhere near full. Found a quick solution to temporarily override the system long enough to flush the toilet. DW will turn the battery disconnect switch (salesman switch as some call it) off for a few seconds. I stand right next to the toilet and when she turns it back on I flush the toilet as soon as the flush controller back light comes on. It seems the system has to "think about it" before it turns the tank full indicator to red.
Ken & Ara Brewer
2020 Newmar Ventana 3407 DP. Freightliner All electric, Cummins B360, Allison. 2014 V-6 Chevy Equinox Toad.