Black water gage stuck at 3/4 full

And we never used the tank for #2 nor did we put paper into it.  Those button sensors are utterly worthless.  Now, for our new trailer, we follow the practice of no paper and no #2, unless it cannot be avoided and we never believe the level indicator.  Just keeping a kind of running guess about how much you've put into it is going to be the best you can get, unless you get one of those really expensive tank monitoring systems, that may or may not work.  I call it the Zen of tank maintenance.

See this YouTube video to show why ice in the tank is useless:  https://www.youtube.com/watch?v=iH6acEmqvcw

Water in the tank with a good biological agent to break down the toilet paper and wastes is much more likely to succeed. There are ones like TankTechs Rx, Unique Products RV Digest-It, Happy Campers, etc. These can be purchased from the links or from Amazon.

I agree that using the Black Tank Flush is helpful as the OP pointed out. If one does not have an excessive accumulation of solid matter from using too little water, that should be sufficient. A wand is useful when one buys a used RV where someone did not use good practices and the tank is clogged. Otherwise, good practices will mean that such a wand is not needed.

Good practices are to use plenty of water with #2 coupled with not emptying the tank until it is at least 2/3 full. That is where accurate gauges are helpful, or a good understanding of your water usage along with a visual check if you think it is getting full. Sometimes, you can tell by the changing sound of a flush. A nearly full tank will sound different than an empty or nearly empty tank.

I can see the level in the fresh water tank by looking under the camper at it. I can see the level in the black tank by looking down the hole. That leaves an unknown level in the gray tank.  

I fill the fresh tank whenever I drain the gray. Since the fresh and gray tanks are the same size, when the fresh is empty the gray will inevitably be full less the small amount that went in the black tank from flushes or on the ground from using the outside shower. Time to empty. I could overfill the gray tank using this method if I used the city water connection, so I only pump from the fresh tank. I don't want more total water weight in the trailer anyway. 

I never look at the little level lights, run out of water without knowing it or overflow a tank. Once you've been camping a few times you pretty much know how much water you're using anyway. I know I can go about 2 weeks by myself between dump stops or about 5 days with my wife.  She takes slightly longer showers than I do. 

Originally posted by scolesworthy I got the answer from a youtube video.  Fill up the tank with fresh water using the back water hose connection.  Then drain.  It worked.  Must have cleaned off something on the sensor.  Thank you for the replys.

Even so, that is only temporary. I have had an ongoing battle with this since we purchased our RP179 in December, 2015. I can get it working, but that only lasts until we use it again. Then it get stuck again.

The SeeLevel system is much better since it reads the level though the tank wall and does not rely on sensors stuck into the tank. It is much more expensive though which is why we don't see them on the RPods with their price point. Another solution would be to replace the tank probes with these: Horst Miracle Probe. These would be used in the black tank. Another model is used for the gray tank that does not have the shield. I have found that the fresh water tank is okay as is.