I really enjoy posts about sensors acting up since I have none.
I check the black tank by shining a flashlight down the toilet while holding the foot pedal down. But I have to be quick since I don't turn the water valve off - the turbulence makes the level easier to judge.
I go outside and lift the portable gray water tank to judge its level since I do not have one built in to my '67 Airstream.
The lower sight glass fitting on my fresh water tank froze & broke around 1972.
But with the problems you guys have, I have yet to experience sensor envy