Easy to replace........just a rubber bung that slips into hole in tank and then the bung is compressed as you tighten screw. That's the easy part.
Access to them is the hard part.......
Underbelly of trailer covered.then that has to be removed
Tank installed up high, next to frame, next to other stuff......can be hard to get to the probes (factory guys get to install them prior to tank going into place)
Probes get cruddy.......always showing level.
4 probes
2 lower ones......ground probe and 1/3
middle probe.......2/3
top probe.........full

There are other styles available......don't protrude thru tank wall, $$$ and you still have to have access
I haven't looked at our tank level monitor for over 6 yrs. Don't care.
Toilet burbs....time to dump
Sink/shower start to drain slower than normal....time to dump