After our last trip we dumped our tanks like always. Plenty came out. On our next trip the black water tank shows 3/4 full and our grey 1/3 full before even used them at all. I’m assuming (hoping) it’s a malfunction on the monitoring system. Is there a way to know without pulling up and moving to the dump station and trying to “empty” again? Has anyone had a similar problem?

You probably have "sludge" on the inside walls of the tanks completing an electrical path between the sensors (the same way actual contents would) that causes the monitor to show a fuller than actual condition.

You can try washing out the black tank to see if that helps.

As I said in another thread, I replaced my sensors in those tanks with Horst Miracle Probes, and I've not had any more false readings.

- Jack

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Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan, LiFePO4 battery 12V DC Electrical System, SoftStartRV mounted on A/C
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Thanks for the response... I should have looked a little more through old threads before posting. My question is: is there a way to know if it is a sensor malfunction or could it be I didn’t actually get it drained last time (clogged outlet)? Is there a simple way to check the level?

If your area has a dump site nearby, you could fill part way and go dump it. If your lucky and you house has a clean-out, you could dump there. Again, if your near an open field you could dump out there. Or trust yourself and with an appropriate sized bucket, open the dump valve and hope only a little bit comes out.
I used the Horst probes as well, however I learned that I have a wiring issue, so I stay clued in as to my gray water level (we do our other business at the rest rooms). When the grey water gets too high, I drain 5-10 gallons out and "put out my fire that night".

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Treeclimber
2703 Tow Lite
2002 Escalade
Bonita Springs, Fl. &
Andrews, NC

I've seen, and used this tip I saw on another RV forum:

Next time you're heading out with the RV, toss a couple of bags of crushed ice into the blackwater tank (via toilet) and drive. The action of the ice will scrub the sludge off the sensors, and the ice will melt in a short time, ready to be drained.

I tried this and it worked to restore my sensors to accuracy. Unfortunately, I haven't figured out how to get a bag of ice into the gray water tank!

Fill the tank about 1/4 way with water and then put the bags of ice in, then squirt about half a bottle of dawn in each tank and drive around a while. The greywater tank will just have water and dawn but it should still sloush enough to help clean it. Then a good rinse or backflush at the dump station should clean them out. I would then again fill both tanks about halfway with fresh water and do the drive and dump thing one more time.

Should do the job.

Hi! Put a bunch of ice cubes in the tank with a bunch of clean water and drive it around till they knock the crud off your sensors. Worked for me! John