[BT on Van Isle]

Okay, all you experienced Montana folks, here is a question I cannot seem to find an answer to. Hoping you can shed some light on this one for me. (I'm a new owner of a 2020 Montana, living in it full time but not moving around for this first year at least.)

My black water tank sensors stopped working correctly almost immediately and since then I've been experimenting with what might work to correct the situation. But even if they never work properly again, my question remains.
Namely: How, if the sensors don't work, do you know when the black tank is nearing full while cleaning/flushing?

I've used a timer for five minutes of back flushing and repeated this twice. I also now have one of those wands with the spinning ends to use from the upper end of the tank (the toilet) but some posts say a fifth wheel's tanks make these wands less effective due to distances from toilet to tank, etc.
Whatever I'm doing -- water flushing the tank from below or above -- I have no non-scary way to know when the tank is nearing full. I've read that you can tell by the sound of the water in the toilet as you fill from that end. I always chicken out.

My black tank sensor is stuck at the 2/3 mark generally; this week they register full, right after emptying and cleaning. So I figure, the dirty sensor must be up near the top of the tank somewhere.

I'm not moving the RV around to jostle the tank contents that way. So creative cleaning and flushing is where it's at for me.

Long story short: Sorry to be repetitive, but how do you get the black tank to near-full without 'going too far'? Thank you!