Should be easy to figure out.......
Watch toilet bowl and see if water is leaking into it.
Bowl seal is not holding water so you should be able to see water build up on seal.
Toilet water valve would have to be leaking thru quite a bit for black tank to get 2/3 full over night.
A drip drip drip would not do that.....a steady stream with a bowl seal not holding would.
And if that is the case...good thing bowl seal doesn't hold water or your toilet would have overflowed and flooded trailer
Just for giggles.........do you have a black tank flush connection and is a hose connected???
Toilet/bathroom sink drain/flush connection....only way for water to black tank