(If you have normal RV plumbing, not the fancy "water bay" or a lever like a submarine handle pointing to different plumbing)
I had one toilet that if you didn't release the flush valve (water supply) smartly, the water valve didn't seat right and water leaked slowly into toilet. Once toilet bowl becomes full, it overflows into waste tank, then when waste tank gets filled up you overflow. No where else for water to go, so you overflow.
Hope I explained it right. The water valve didn't shut off the water supply IMO. In ours we thought the toilet water supply valve was bad. Got a new one. In process of changing the water valve, noticed some plastic parts also worn. Changed the valve and couple plastic parts that slide against each other and had become worn. Then no more problems.
That's one reason we don't use the city water connection anymore. If you were in the self contained mode, your water pump would of been making a burping noise and you might of been able to hear it pushing water, before it overflowed.
With city water, I can't here water running when using city water supply and a lot more water available than my fresh water tank.
2003 Majestic 23P... Northwest travel machine
2013 Arctic Fox 25W... Wife "doll house" for longer snowbird trips
2001 "The Mighty Dodge"... tow vehicle for "doll house"