I have had 2 campers, 4 trailers, and 1 5th wheel over 35 years so I have been around. However, I am puzzled!
I drained my black water (which read full) and saw 2/3 full after draining. I added water and drained again. Still reads full. So I then use the interior spray and add 10 gallons (approx) - and drain. And see FULL. Funny thing is, I can see and hear a LOT of black water going down into the dump station.
OK. So I fill it up with 3 gallons of water (through the toilet) and drive home (5 miles including a rough road). I get home and it reads 2/3 full! Yikes! Today, 3 days later, it still reads 2/3 full.
Any ideas as to what is going on? I would guess blockage somewhere, but I have pretty much followed correct procedure.
Steve and Bobbie (and Shotgun - my 115 lb dog)
Down sized to a Nash 25S
My web page is www.dustylens.com
Sensors get yucky--I wonder why. Not using enough water in the black tank exacerbates the problem. I put a cup of Calgon in our black tank once or twice a season. Then I drive 100 miles or more on winding mountain roads before dumping. You can also dump a bag of ice cubes into the toilet and drive to clean the sensors.
I installed SeeLevel gauges on the DP; we'll see how long we go with the OE sensors before I do the same on the TT.
Steve & C. J.
"Gracie" the Rough Collie & "Bo'sun" the Bichon Frise
I've heard of some folks putting a few bags of crushed ice in the black tank. Then drive around and agitate it. It's supposed to scrub the tank and probes.
Most tank gauges are unreliable JUNK.. You should know how long you can poop your before you tank needs dumping. It takes the 2 of us about 2 weeks of chiliburgers before ours is near the dumping point (40 gallons). Gray water tanks take a little more finess and require more frequent dumps. I always dump black before leaving a park and always try to hit the road with empty bilge tanks. If I had to schedule a Honeybucket truck I'd ask for once a week dumps.
2005 Dutchmen 35SRV, Pergo floor, 5th Airborne, JT Strong Arm, Kodiak Disk Brakes, Backup camera
2006 Dodge CTD RAM QC 3500 LWB 4WD, Reese 16k, Airlift, Pacbrake, Britebox, 60g aux fuel tank
04 HD Ultra, CanAM 400MAX Quad
We had the same problem on our last trip. Had some water and blue stuff in the tank for the ride home. 2 days ago it still read 2/3. Today it is finally down to 1/3.
Thanks folks. I suspected TP hanging on a sensor. Never tried the ice cube OR fabric softener trick. I did try adding more water with more blue stuff, and driving around - unsuccessfully.
All it takes is a very small piece ot TP or ..... to stay on the sensor and it will read full. Sometimes they need to dry out a little. Please follow the geo method and backfill with a Flush King until the water runs clear everythime you use your trailer. I promise your sensors will work correctly and your tanks will stay clean. When I am done dumping I put 5-10 gal of water back in each tank with 1/2 gal of bleach and drive home, I then dump again and backfill a couple more times. Yes I am anal about my tanks, but they are clean and the sensors always work.
NCH
2000 Ford F350 4X4 PSD,CC, DRW
4:10's, BTM Muffler, Isspro Gauges,
Coolant Filter,CCV Mod
FTVB , Ford AIS
6.0 Cooler,SCT2 W/DP's 40 tow & 80 econo,Zoodad mod
2005 Keystone Hornet 30BHSS
Reese Dual Cam
Family of 4 saved by Grace!!! MY TRUCK OUR TRAILER
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Not using enough water in the black tank exacerbates the problem. I put a cup of Calgon in our black tank once or twice a season. Then I drive 100 miles or more on winding mountain roads before dumping. You can also dump a bag of ice cubes into the toilet and drive to clean the sensors. 
