Try filling the black tank about 2/3 full then go for a drive on a curvy bumpy road if you can.  When you get back drain and flush.  I've also been told to put lots of ice cubes in the tank just before you drive it.  The cubes will scrub the walls and sensors. If you have a vacu flush system, like I do, ice won't fit into the tank.  There are also tank cleaning companies that will pressure wash it inside of the tank.  I've had that done and it works great and cleans the sensors.  I have also just lived with the sensor stuck for awhile until I get somewhere I can take care of it.  Good luck.

Good luck on that, ours always shows 3/4 full even after Decatur said they fixed.  We have done everything that others told us and no results.  I just bought Commando and have not had a chance to use it yet, but will report on our findings.  We have a 2014 Rev with macerator and vacuum flush commodes so not able to put ice in the tanks.

I had the same issue on my 2015 Bounder 35k. Can't use ice due to the vacuum flush toilets. I purchased some enzyme cleaner at the FMCA Rally last summer and used it. I also modified my flush procedure by dumping the black tank, starting the flush and closing the black tank valve for about several minutes (not enough to fill the tank). I stopped the flush water flow and opended the valve for a final dump. Cleaned it out quite well and the level indicator issue was resolved. It's happened a couple of other times, but this process helped each time.

just a update on our tank reading.  Always 3/4 full.  Bought Commando and put in black tank yesterday and filled with water.  Says to leave in 12 to 24 hours.  We drove 160 miles and dumped when arriving at park, hooray, empty and green lights at both commodes.  Hope this keeps us clean.