Tank guage lies

Wizened old timers tip #10,371. If the black water tank is directly under the toilet, turn the water off, flush the toilet and shine a flashlight down into the tank, reveling the level.

My gauges do not work, I have the time, knowledge and tools to fix them, it is not worth it.

Since you are on a fixed site, and you have exceptionally hard water in Tucson, I guess you're having the RV version of septic caliche in your tank. The stationary water level with no sloshing has allowed the organic material to bind with the minerals in the water creating scale the likes only those in the SW desert can understand.

Solutions: Blast it away. Use a industrial septic cleaner. Try those foaming RV tank cleaners like Tank Blaster or Commando. Fill tank with water and CLR or Lime Away and rock the tank from time to time. Fill the tank with vinegar and rock the tank from time to time.

I suspect a contributing factor is your tank isn't getting sloshed causing minerals to settle where they are, allowing other minerals and waste to just build on top of each other. For what it's worth, I know many with the exact same issue when their unit is in used but sitting fixed for a long time.

Good luck.

Yep, try this, try that, try the next thing, and when you spent way too much money and time, then you do like we do. Anymore, I doubt I'd use them if they did work.

My Quickie-Flush tank rinser with custom spray pattern seems to keep mine working just fine.

before you head north this summer you can try this
fill with clean water and put in 'waste digester' used for septic systems
it takes months to work but will eat the waste

then when you come back next winter, rinse multiple times with clear water

for now you might try a power rinse tool/nozzle
one of those things you stick down into the toliet
a lot of people whose campers have bathroom windows (to bring a hose thru) use them

Well... there are aftermarket sensors that are not cheap, or you can get a spray wand, shove it down the commode, and let 'er rip. You can try the calgon/ice/drive around thing.

Or you can do what most of us do, ignore the useless lights and just figure out how full the tank is by usage and time. After a while it's easy.

Ignore them, they're always like that. You'll find you can judge the tanks fullness by time and use.
I haven't checked my tanks since the first day I had the RV. Then I just made sure they worked.

They hardly ever work. You can try adding calgon and dawn detergent, soaking overnight. Then draining may take several applications. Some people say dumping ice cubes into the tank and then driving around make beat the deposits off.

There are better sensors you can go with. Amazon. I see a sensor cleaner there too.

If you click on that cleaner item you'll see other tank cleaners. I don't trust anything with 4 reviews.