Fred.....The Dometic system uses a pad that attaches to the tank to tell the toilet to stop flushing when it's full. Your pad might have fallen off and is laying face down.
If you look, you'll also find a small black box, about the size of a deck of cards that controls your toilet. My coach has two toilets and once the red light comes on, the toilets quit flushing. So.......I opened the Dometic box, described above and cut the orange wire. This permanently turns off the red light and allows the toilet to flush. For us, once one of the toilets goes to red, I know I can still flush the other one at least 3-5 more times. This is important in the middle of the night.
I put a switch on the orange wire I cut, so I could turn the red light back on. Once you get the light issue fixed. it might be handy to install a switch like I did, run to the interior of the coach, so you could still use the toilet late at night. The controller on my mid bath was in an outside bay below the toilet. On my rear toilet, it was a behind a panel near the toilet.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab