I am really sorry your experience with them was not as good as mine (so far).
Some things to check (assuming you want to drop your belly pan again) is the lowest probe being oriented correctly. I almost rotated mine out of alignment when I tightened up the wire nut. I wear bifocals and of course the focal length at arm's length sucks. I can read and drive fine, but shooting a rifle or working over my head with tools is a challenge. Trying to fine focus at that distance is darn near impossible.
The only way for that probe to be "on" is for there to be an electrical connection between the probe wire and the ground probe. If you are not getting a complete dump for any reason the "1/3" probe will be under water. Since the ground and the "on" probe are at the same level in the tank, it won't take much to turn it on legitimately. The dump valve is not at the very bottom either. Some water will always be in the tank unless you let it dry out with the valve open.
In my camper, one #1 flush will turn on that light.
It is really a "Not Empty" light.
As you can see from the picture of the tank, it will not take much sludge in the bottom of the tank to keep that light on. Maybe an RV-Digest-it treatment for 24-48 hours might be in order.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW