For water tank senders, I am using WEMA brand devices. Considered by many to be very reliable.
For black water, in our 28 footer we tried the cheap Xintex/Firebuoy tank monitors but had all sorts of problems with it before it stopped working altogether. Not recommended.
I think the best choice for a black water tank monitor would be an ultrasonic type sender. No moving parts and nothing to foul. But they are not cheap. I have no experience with them either.